Semiconductor device and method for manufacturing the same

ABSTRACT

A transistor excellent in electrical characteristics and a method for manufacturing the transistor are provided. The transistor includes an oxide semiconductor layer including a source region, a drain region, and a channel formation region over an insulating surface; a gate insulating film over the oxide semiconductor layer; a gate electrode overlapping with the channel formation region, over the gate insulating film; a source electrode in contact with the source region; and a drain electrode in contact with the drain region. The source region and the drain region include a portion having higher oxygen concentration than the channel formation region.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a semiconductor device and a method formanufacturing the semiconductor device.

Note that in this specification, a semiconductor device refers to anydevice that can function by utilizing semiconductor characteristics, andan electro-optical device, a semiconductor circuit, and an electronicdevice are all included in the category of semiconductor devices.

2. Description of the Related Art

Attention has been focused on a technique for forming a transistor usinga semiconductor thin film formed over a substrate having an insulatingsurface (also referred to as thin film transistor (TFT)). The transistoris applied to a wide range of electronic devices such as an integratedcircuit (IC) and an image display device (display device). Asilicon-based semiconductor material is widely known as a material for asemiconductor thin film applicable to a transistor. As another material,an oxide semiconductor material has been attracting attention.

For example, a transistor whose active layer includes an amorphous oxidesemiconductor containing indium (In), gallium (Ga), and zinc (Zn) isdisclosed (see Patent Document 1).

REFERENCE Patent Document

-   [Patent Document 1] Japanese Published Patent Application No.    2006-165528

SUMMARY OF THE INVENTION

It is known that oxygen vacancy in an oxide semiconductor and hydrogencontained therein as an impurity serve as donors; thus, in the casewhere an oxide semiconductor is used for a channel formation region of atransistor, an oxide semiconductor layer having as little oxygenvacancy, hydrogen, and moisture as possible is preferably used. However,oxygen is desorbed from the oxide semiconductor layer through heattreatment performed as dehydration or dehydrogenation treatment on theoxide semiconductor layer or an insulating film in contact with theoxide semiconductor layer.

Desorption of oxygen from the oxide semiconductor layer becomes a factorof a change in electrical characteristics of a transistor, and thusoxygen vacancy due to desorption of oxygen from the oxide semiconductorlayer needs to be filled. For that reason, development of a method forefficiently supplying oxygen to the oxide semiconductor layer has beenrequired.

In view of this, an object of one embodiment of the present invention isto provide a transistor in which oxygen is efficiently supplied to achannel formation region and which has excellent electricalcharacteristics. Another object is to provide a method for manufacturingthe transistor.

One embodiment of the present invention disclosed in this specificationrelates to a transistor in which the oxygen concentration in a sourceregion and a drain region is higher than that in a channel formationregion, and relates to a method for manufacturing the transistor.

One embodiment of the present invention disclosed in this specificationis a semiconductor device including an oxide semiconductor layerincluding a source region, a drain region, and a channel formationregion over an insulating surface; a gate insulating film over the oxidesemiconductor layer; a gate electrode overlapping with the channelformation region, over the gate insulating film; a source electrode incontact with the source region; and a drain electrode in contact withthe drain region. The source region and the drain region include aportion having higher oxygen concentration than the channel formationregion.

In the oxide semiconductor layer, the channel formation regionpreferably includes a c-axis aligned crystal, and the portion which isin the source region and the drain region and has higher oxygenconcentration than the channel formation region is preferably amorphous.

An impurity for improving conductivity of the oxide semiconductor layeris preferably added to the portion which is in the source region and thedrain region and has higher oxygen concentration than the channelformation region.

At least one of the gate electrode, the source electrode, and the drainelectrode may be electrically connected to a semiconductor deviceincluding a semiconductor layer whose band gap is different from a bandgap of the oxide semiconductor layer.

An insulating film containing aluminum oxide is preferably formed overthe gate insulating film and the gate electrode.

Another embodiment of the present invention disclosed in thisspecification is a method for manufacturing a semiconductor device,including the sequential steps of preparing a substrate having aninsulating surface; forming an oxide semiconductor layer over theinsulating surface; forming a gate insulating film over the oxidesemiconductor layer; forming a gate electrode over the gate insulatingfilm so as to overlap with the oxide semiconductor layer; adding oxygento a region which is in the oxide semiconductor layer and does notoverlap with the gate electrode; adding an impurity to the region whichis in the oxide semiconductor layer and does not overlap with the gateelectrode to form a source region, a drain region, and a channelformation region; forming an insulating film over the gate insulatingfilm and the gate electrode; performing heat treatment on the oxidesemiconductor layer; and forming a source electrode in contact with thesource region and a drain electrode in contact with the drain region.

Another embodiment of the present invention disclosed in thisspecification is a method for manufacturing a semiconductor device,including the sequential steps of preparing a substrate having aninsulating surface; forming an oxide semiconductor layer over theinsulating surface; forming a source electrode and a drain electrode incontact with the oxide semiconductor layer; forming a gate insulatingfilm over the oxide semiconductor layer, the source electrode, and thedrain electrode; forming a gate electrode over the gate insulating filmso as to overlap with the oxide semiconductor layer; adding oxygen to aregion which is in the oxide semiconductor layer and does not overlapwith the gate electrode, the source electrode, or the drain electrode;adding an impurity to the region which is in the oxide semiconductorlayer and does not overlap with the gate electrode, the sourceelectrode, or the drain electrode to form a source region, a drainregion, and a channel formation region; forming an insulating film overthe gate insulating film and the gate electrode; and performing heattreatment on the oxide semiconductor layer.

In the two methods for manufacturing the semiconductor device, the stepof adding the impurity to the oxide semiconductor layer may be performedbefore the step of adding oxygen to the oxide semiconductor layer, afterthe step of forming the insulating film, or after the step of performingthe heat treatment on the oxide semiconductor layer.

Another embodiment of the present invention disclosed in thisspecification is a method for manufacturing a semiconductor device,including the steps of preparing a substrate having an insulatingsurface; forming a source electrode and a drain electrode over theinsulating surface; forming an oxide semiconductor layer in contact withthe source electrode and the drain electrode; forming a gate insulatingfilm over the source electrode, the drain electrode, and the oxidesemiconductor layer; forming a gate electrode over the gate insulatingfilm so as to overlap with part of the source electrode, part of thedrain electrode, and part of the oxide semiconductor layer; addingoxygen to a region which is in the oxide semiconductor layer and doesnot overlap with the gate electrode; forming an insulating film over thegate insulating film and the gate electrode; and performing heattreatment on the oxide semiconductor layer.

In any of the methods for manufacturing the semiconductor device, theinsulating film is preferably an insulating film containing aluminumoxide.

According to one embodiment of the present invention, a transistor inwhich oxygen can be efficiently supplied to a channel formation regionand which has excellent electrical characteristics can be provided.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A and 1B are a top view and a cross-sectional view illustrating asemiconductor device according to one embodiment of the presentinvention.

FIGS. 2A and 2B are a top view and a cross-sectional view illustrating asemiconductor device according to one embodiment of the presentinvention.

FIGS. 3A and 3B are a top view and a cross-sectional view illustrating asemiconductor device according to one embodiment of the presentinvention.

FIGS. 4A to 4D illustrate a method for manufacturing a semiconductordevice according to one embodiment of the present invention.

FIGS. 5A to 5D illustrate a method for manufacturing a semiconductordevice according to one embodiment of the present invention.

FIGS. 6A to 6D illustrate a method for manufacturing a semiconductordevice according to one embodiment of the present invention.

FIGS. 7A and 7B are a cross-sectional view and a circuit diagramillustrating one embodiment of a semiconductor device.

FIGS. 8A and 8B are a circuit diagram and a perspective viewillustrating one embodiment of a semiconductor device.

FIG. 9A is a block diagram illustrating a structure example of a CPU,and FIGS. 9B and 9C each illustrate a configuration example of part ofthe CPU.

FIGS. 10A to 10C each illustrate an electronic device.

FIGS. 11A to 11C illustrate an electronic device.

FIGS. 12A to 12C illustrate electronic devices.

FIGS. 13A to 13C are model diagrams used for calculation of movement ofexcessive oxygen.

FIG. 14 shows calculation results of the models in FIGS. 13A to 13C.

FIGS. 15A to 15C are model diagrams used for calculation of movement ofoxygen vacancy.

FIG. 16 shows calculation results of the models in FIGS. 15A to 15C.

DETAILED DESCRIPTION OF THE INVENTION

Embodiments of the present invention will be described in detail withreference to the accompanying drawings. Note that the present inventionis not limited to the following description and it will be readilyappreciated by those skilled in the art that modes and details can bemodified in various ways without departing from the spirit and scope ofthe present invention. The present invention therefore should not beconstrued as being limited to the following description in theembodiments. Note that in structures of the present invention describedbelow, the same portions or portions having similar functions aredenoted by the same reference numerals in different drawings, anddescription thereof is not repeated in some cases.

Embodiment 1

In this embodiment, a semiconductor device according to one embodimentof the present invention and a method for manufacturing thesemiconductor device will be described.

FIG. 1A is a top view of a transistor according to one embodiment of thepresent invention, and FIG. 1B is a cross-sectional view taken alongline A1-A2 in FIG. 1A. Note that some components are not illustrated inFIG. 1A for simplicity.

A transistor 191 in FIGS. 1A and 1B includes a base insulating film 110formed over a substrate 100; an oxide semiconductor layer 120 formedover the base insulating film; a gate insulating film 130 formed overthe oxide semiconductor layer; a gate electrode 140 formed over the gateinsulating film; a protective film 160 formed over the gate insulatingfilm and the gate electrode; a planarization film 170 formed over theprotective film; and a source electrode 150 a and a drain electrode 150b in contact with the oxide semiconductor layer through contact holesformed in the protective film and the planarization film. Note that theprotective film 160 and the planarization film 170 may be provided asneeded.

FIGS. 1A and 1B illustrate an example of a self-aligned top-gatetransistor that can be used in one embodiment of the present invention;the kinds, shapes, and positional relationships of components are notlimited to those illustrated in FIGS. 1A and 1B.

Functions of a “source” and a “drain” of a transistor are sometimesreplaced with each other when a transistor of opposite polarity is usedor when the direction of current flowing is changed in circuitoperation, for example. Therefore, the terms “source” and “drain” can beused to denote the drain and the source, respectively, in thisspecification.

The substrate 100 is not limited to a simple supporting substrate, andmay be a substrate where a device such as a transistor is formed. Inthis case, at least one of the gate electrode 140, the source electrode150 a, and the drain electrode 150 b of the transistor 191 may beelectrically connected to the device.

The base insulating film 110 can have a function of supplying oxygen tothe oxide semiconductor layer 120 as well as a function of preventingdiffusion of an impurity from the substrate 100; thus, the baseinsulating film 110 is preferably an insulating film containing oxygen.Note that in the case where the substrate 100 is a substrate whereanother device is formed as described above, the base insulating film110 has also a function as an interlayer insulating film. In that case,the base insulating film 110 is preferably subjected to planarizationtreatment such as chemical mechanical polishing (CMP) treatment so as tohave a flat surface.

The oxide semiconductor layer 120 is processed into an island shape, andoverlaps with the gate electrode 140 with the gate insulating film 130therebetween. In the oxide semiconductor layer 120, a region whichoverlaps with the gate electrode 140 is a channel formation region 120a, and a region which does not overlap with the gate electrode 140 is asource region or drain region 120 b.

The channel formation region 120 a is formed using an oxidesemiconductor including crystals with c-axis alignment. Here, thecrystals with c-axis alignment mean crystals whose c-axes of crystalaxes are aligned in the direction parallel to a normal vector of asurface where the film is formed or a normal vector of a surface of thefilm.

On the other hand, the source region or drain region 120 b is amorphous.The amorphous source region or drain region 120 b includes a largenumber of defects and the like serving as gettering sites, and thus cangetter hydrogen, moisture, and the like in the channel formation region120 a, the base insulating film 110, and the gate insulating film 130.Further, the amorphous source region or drain region 120 b can getterhydrogen, moisture, and the like which are to enter the channelformation region 120 a in the manufacturing process or operation of thetransistor.

Here, the source region or drain region 120 b contains a larger amountof oxygen than the channel formation region 120 a. When excessive oxygenin the source region or drain region 120 b is diffused into the channelformation region 120 a, oxygen vacancies and the like in the channelformation region 120 a which are caused owing to a heating step or thelike in the manufacturing process of the transistor can be filled.Further, oxygen vacancies and the like in the channel formation region120 a which are caused owing to long-time operation or operationenvironment of the transistor can also be filled.

In the channel formation region 120 a formed using an oxidesemiconductor including crystals with c-axis alignment, oxygen atomseasily move in the horizontal direction (direction substantiallyperpendicular to the c-axes); thus, oxygen can be efficiently diffusedfrom the direction of the source region or drain region 120 b to thechannel formation region 120 a. Note that the channel formation region120 a is preferably in an oxygen-excess state where the oxygen contentis in excess of that in the stoichiometric composition.

An impurity for improving the conductivity of the oxide semiconductorlayer is preferably added to the source region or drain region 120 b. Asthe impurity, for example, one or more selected from the following canbe used: phosphorus (P), arsenic (As), antimony (Sb), boron (B),aluminum (Al), nitrogen (N), argon (Ar), helium (He), neon (Ne), indium(In), fluorine (F), chlorine (Cl), titanium (Ti), zinc (Zn), and carbon(C).

Over the gate insulating film 130 and the gate electrode 140, aninsulating film containing aluminum oxide is preferably formed as theprotective film 160. The aluminum oxide film has a high blocking effectof preventing penetration of both oxygen and an impurity such ashydrogen or moisture. Accordingly, the aluminum oxide film can besuitably used as a protective film that prevents entry of an impuritysuch as hydrogen or moisture, which causes variation in the electriccharacteristics of the transistor, into the oxide semiconductor layer120 and release of oxygen, which is a main component material of theoxide semiconductor layer 120, from the oxide semiconductor layer duringand after the manufacturing process. Note that another insulating filmmay be formed between the protective film 160, and the gate insulatingfilm 130 and the gate electrode 140.

Note that a transistor that can be used in one embodiment of the presentinvention may have a structure illustrated in FIGS. 2A and 2B.

FIG. 2A is a top view of a transistor, and FIG. 2B is a cross-sectionalview taken along line B1-B2 in FIG. 2A. Note that some components arenot illustrated in FIG. 2A for simplicity.

A transistor 192 in FIGS. 2A and 2B includes the base insulating film110 formed over the substrate 100; the oxide semiconductor layer 120formed over the base insulating film; the source electrode 150 a and thedrain electrode 150 b formed in contact with the oxide semiconductorlayer; the gate insulating film 130 formed over the oxide semiconductorlayer, and the source electrode and the drain electrode; the gateelectrode 140 formed over the gate insulating film; the protective film160 formed over the gate insulating film and the gate electrode; and theplanarization film 170 formed over the protective film. Note that theprotective film 160 and the planarization film 170 may be provided asneeded.

The transistor 192 is different from the transistor 191 in the positionsof the source electrode 150 a and the drain electrode 150 b.Accordingly, in the oxide semiconductor layer 120, a region 120 c isformed in addition to the channel formation region 120 a and the sourceregion or drain region 120 b.

The source region or drain region 120 b is formed by addition of oxygenor an impurity for improving conductivity by an ion implantation methodor the like after the formation of the gate electrode 140. In thetransistor 191, the gate electrode 140 serves as a mask; thus, theimpurity for improving the conductivity is added to regions in the oxidesemiconductor layer 120 which do not overlap with the gate electrode140, so that the source region or drain region 120 b is formed. On theother hand, in the transistor 192, the source electrode 150 a and thedrain electrode 150 b also serve as masks, so that the region 120 c isformed in the oxide semiconductor layer 120.

Although the impurity for improving the conductivity is not added to theregion 120 c as well as to the channel formation region 120 a, theresistance of the region 120 c is negligible because the region 120 c isin contact with a metal material of the source electrode 150 a and thedrain electrode 150 b. Therefore, it can be said that the region 120 cis part of the source region or drain region.

Note that a transistor that can be used in one embodiment of the presentinvention may have a structure illustrated in FIGS. 3A and 3B.

FIG. 3A is a top view of a transistor, and FIG. 3B is a cross-sectionalview taken along line C1-C2 in FIG. 3A. Note that some components arenot illustrated in FIG. 3A for simplicity.

A transistor 193 in FIGS. 3A and 3B includes the base insulating film110 formed over the substrate 100; the source electrode 150 a and thedrain electrode 150 b formed over the base insulating film; the oxidesemiconductor layer 120 formed in contact with the source electrode andthe drain electrode; the gate insulating film 130 formed over the sourceelectrode, the drain electrode, and the oxide semiconductor layer 120;the gate electrode 140 formed over the gate insulating film; theprotective film 160 formed over the gate insulating film and the gateelectrode; and the planarization film 170 formed over the protectivefilm. Note that the protective film 160 and the planarization film 170may be provided as needed.

The transistor 193 is different from the transistor 191 and thetransistor 192 in that the source electrode 150 a and the drainelectrode 150 b partly overlap with the gate electrode 140. Accordingly,the channel formation region 120 a is in contact with the sourceelectrode 150 a and the drain electrode 150 b; thus, a step of adding animpurity to the source region or drain region 120 b for loweringresistance thereof may be skipped.

Next, a method for manufacturing the transistor 191 in FIGS. 1A and 1Bwill be described with reference to FIGS. 4A to 4D.

First, the base insulating film 110 is formed over the substrate 100.There is no particular limitation on a material for the substrate 100 aslong as it withstands a heating step performed later. For example, aninsulating substrate such as a glass substrate, a semiconductorsubstrate such as a silicon wafer, or the like can be used.Alternatively, a substrate where another device is formed may be used asdescribed above.

The base insulating film 110 can be formed by a plasma CVD method, asputtering method, or the like using an oxide insulating film of siliconoxide, silicon oxynitride, aluminum oxide, aluminum oxynitride, hafniumoxide, gallium oxide, or the like; a nitride insulating film of siliconnitride, silicon nitride oxide, aluminum nitride, aluminum nitrideoxide, or the like; or a mixed material of any of these. Further, alayered structure including any of the above materials may be employedfor the base insulating film 110; in that case, at least an upper layerin contact with the oxide semiconductor layer 120 is preferably formedusing a material containing oxygen so that oxygen can be suppliedtherefrom to the oxide semiconductor layer 120.

Next, an oxide semiconductor film is formed over the base insulatingfilm 110 and then processed into an island shaped by a photolithographymethod and an etching method to form the oxide semiconductor layer 120(see FIG. 4A).

After the oxide semiconductor film is formed, heat treatment forreducing or removing excess hydrogen (including water and a hydroxylgroup) in the oxide semiconductor film (dehydration or dehydrogenation)is preferably performed. The temperature of the heat treatment is higherthan or equal to 300° C. and lower than or equal to 700° C. In the casewhere a glass substrate is used as the substrate 100, the temperature ofthe heat treatment is lower than the strain point of the substrate. Theheat treatment is preferably performed under reduced pressure, anatmosphere of an inert gas such as nitrogen or a rare gas, or anatmosphere containing oxygen.

The heat treatment enables hydrogen, which is an impurity impartingn-type conductivity, in the oxide semiconductor film to be reduced orremoved. Further, in the case where an insulating layer containingoxygen is used as the base insulating film 110, by this heat treatment,oxygen contained in the base insulating film 110 can be supplied to theoxide semiconductor film. By supplying oxygen from the base insulatingfilm 110, oxygen vacancies in the oxide semiconductor film increased bythe dehydration or dehydrogenation treatment can be filled.

The heat treatment for the dehydration or dehydrogenation may beperformed after the island-shaped oxide semiconductor layer 120 isformed. Further, the heat treatment for the dehydration ordehydrogenation may serve as another heat treatment in the manufacturingprocess of the transistor.

Note that it is preferable that in the heat treatment, moisture,hydrogen, and the like be not contained in nitrogen, oxygen, or a raregas such as helium, neon, or argon. It is also preferable that thepurity of the above gas be set to 6N (99.9999%) or higher, preferably 7N(99.99999%) or higher (that is, the impurity concentration is 1 ppm orlower, preferably 0.1 ppm or lower).

In addition, after the oxide semiconductor film is heated by the heattreatment, a high-purity oxygen gas, a high-purity dinitrogen monoxidegas, or ultra dry air (the moisture amount is less than or equal to 20ppm (−55° C. by conversion into a dew point), preferably less than orequal to 1 ppm, more preferably less than or equal to 10 ppb, in themeasurement with the use of a dew point meter of a cavity ring downlaser spectroscopy (CRDS) system) may be introduced into the samefurnace while the heating temperature is maintained or slow cooling isperformed to lower the temperature from the heating temperature. It ispreferable that water, hydrogen, and the like be not contained in theoxygen gas or the dinitrogen monoxide gas. The purity of the oxygen gasor the dinitrogen monoxide gas which is introduced into a heat treatmentapparatus is preferably 6N or higher, more preferably 7N or higher(i.e., the impurity concentration in the oxygen gas or the dinitrogenmonoxide gas is preferably 1 ppm or lower, more preferably 0.1 ppm orlower). The oxygen gas or the dinitrogen monoxide gas acts to supplyoxygen in order that oxygen vacancies increased by the step of removingan impurity for the dehydration or dehydrogenation can be filled; thus,the oxide semiconductor film can have high purity and be an i-type(intrinsic) oxide semiconductor film.

Alternatively, as a method for supplying oxygen to the oxidesemiconductor film, an ion implantation method, an ion doping method, aplasma immersion ion implantation method, a plasma treatment method, orthe like may be used. In this case, oxygen supply to the oxidesemiconductor film through the gate insulating film 130 to be formedlater may be performed as well as oxygen supply directly to the oxidesemiconductor film.

The introduction of oxygen to the oxide semiconductor film can beperformed anytime after the dehydration or dehydrogenation treatment isperformed thereon. Further, oxygen may be introduced plural times intothe dehydrated or dehydrogenated oxide semiconductor film.

The oxide semiconductor film may be amorphous or may include a crystalcomponent. In the case where an amorphous oxide semiconductor film isused, the amorphous oxide semiconductor film may be subjected to heattreatment in a later manufacturing step so as to be crystallized. Theheat treatment for crystallizing the amorphous oxide semiconductor filmis performed at a temperature higher than or equal to 250° C. and lowerthan or equal to 700° C., preferably higher than or equal to 400° C.,further preferably higher than or equal to 500° C., still furtherpreferably higher than or equal to 550° C. Note that the heat treatmentcan also serve as another heat treatment in the manufacturing process.

The oxide semiconductor film can be formed by a sputtering method, amolecular beam epitaxy (MBE) method, a CVD method, a pulse laserdeposition method, an atomic layer deposition (ALD) method, or the likeas appropriate. Alternatively, the oxide semiconductor film may beformed using a sputtering apparatus where film formation is performedwith surfaces of a plurality of substrates set substantiallyperpendicular to a surface of a sputtering target.

In the formation of the oxide semiconductor film, the hydrogenconcentration in the oxide semiconductor film is preferably reduced asmuch as possible. In order to reduce the hydrogen concentration, forexample, in the case where the oxide semiconductor film is formed by asputtering method, oxygen, a high-purity rare gas (typically, argon)from which impurities such as hydrogen, water, a hydroxyl group, andhydride have been removed, or a mixed gas of oxygen and the rare gas isused as appropriate as an atmosphere gas supplied to a depositionchamber of a sputtering apparatus.

The oxide semiconductor film is formed in such a manner that asputtering gas from which hydrogen and moisture have been removed isintroduced into the deposition chamber while moisture remaining thereinis removed, whereby the hydrogen concentration in the formed oxidesemiconductor film can be reduced. In order to remove moisture remainingin the deposition chamber, an entrapment vacuum pump such as a cryopump,an ion pump, or a titanium sublimation pump is preferably used. A turbomolecular pump provided with a cold trap may be alternatively used. Acryopump has a high capability in removing a hydrogen molecule, acompound containing a hydrogen atom such as water (H₂O) (preferably,also a compound containing a carbon atom), and the like; therefore, theimpurity concentration in the oxide semiconductor film formed in thedeposition chamber which is evacuated using a cryopump can be reduced.

Further, when the oxide semiconductor film is formed by a sputteringmethod, the relative density (filling rate) of a metal oxide target thatis used for the deposition is greater than or equal to 90%, preferablygreater than or equal to 95%. This is because, with the use of the oxidetarget with a high relative density, the formed oxide semiconductor filmcan be a dense film.

In order to reduce the impurity concentration in the oxide semiconductorfilm, it is also effective to form the oxide semiconductor film whilethe substrate is kept at high temperature. The temperature at which thesubstrate is heated may be higher than or equal to 150° C. and lowerthan or equal to 450° C.; the substrate temperature is preferably higherthan or equal to 200° C. and lower than or equal to 350° C. By heatingthe substrate at high temperature during the film formation, acrystalline oxide semiconductor layer can be formed.

An oxide semiconductor used for the oxide semiconductor film preferablycontains at least indium (In) or zinc (Zn). In particular, both In andZn are preferably contained. As a stabilizer for reducing variation inelectrical characteristics of a transistor using the oxidesemiconductor, gallium (Ga) is preferably additionally contained. Tin(Sn) is preferably contained as a stabilizer. Hafnium (Hf) is preferablycontained as a stabilizer. Aluminum (Al) is preferably contained as astabilizer. Zirconium (Zr) is preferably contained as a stabilizer.

As another stabilizer, one or plural kinds of lanthanoid such aslanthanum (La), cerium (Ce), praseodymium (Pr), neodymium (Nd), samarium(Sm), europium (Eu), gadolinium (Gd), terbium (Tb), dysprosium (Dy),holmium (Ho), erbium (Er), thulium (Tm), ytterbium (Yb), or lutetium(Lu) may be contained.

As the oxide semiconductor, for example, indium oxide, tin oxide, zincoxide, an In—Zn-based oxide, a Sn—Zn-based oxide, an Al—Zn-based oxide,a Zn—Mg-based oxide, a Sn—Mg-based oxide, an In—Mg-based oxide, anIn—Ga-based oxide, an In—Ga—Zn-based oxide (also referred to as IGZO),an In—Al—Zn-based oxide, an In—Sn—Zn-based oxide, a Sn—Ga—Zn-basedoxide, an Al—Ga—Zn-based oxide, a Sn—Al—Zn-based oxide, anIn—Hf—Zn-based oxide, an In—La—Zn-based oxide, an In—Ce—Zn-based oxide,an In—Pr—Zn-based oxide, an In—Nd—Zn-based oxide, an In—Sm—Zn-basedoxide, an In—Eu—Zn-based oxide, an In—Gd—Zn-based oxide, anIn—Tb—Zn-based oxide, an In—Dy—Zn-based oxide, an In—Ho—Zn-based oxide,an In—Er—Zn-based oxide, an In—Tm—Zn-based oxide, an In—Yb—Zn-basedoxide, an In—Lu—Zn-based oxide, an In—Sn—Ga—Zn-based oxide, anIn—Hf—Ga—Zn-based oxide, an In—Al—Ga—Zn-based oxide, anIn—Sn—Al—Zn-based oxide, an In—Sn—Hf—Zn-based oxide, or anIn—Hf—Al—Zn-based oxide can be used.

Note that here, for example, an “In—Ga—Zn-based oxide” means an oxidecontaining In, Ga, and Zn as its main component and there is noparticular limitation on the ratio of In:Ga:Zn. The In—Ga—Zn-based oxidemay contain a metal element other than In, Ga, and Zn.

Alternatively, a material represented by InMO₃(ZnO)_(m) (m>0, m is notan integer) may be used as an oxide semiconductor. Note that Mrepresents one or more metal elements selected from Ga, Fe, Mn, and Co.Alternatively, as the oxide semiconductor, a material represented byIn₂SnO₅(ZnO)_(n) (n>0, n is an integer) may be used.

Note that the oxide semiconductor film is preferably formed by asputtering method. As a sputtering method, an RF sputtering method, a DCsputtering method, an AC sputtering method, or the like can be used. Inparticular, a DC sputtering method is preferably used because dustgenerated in the deposition can be reduced and the film thickness can beuniform.

For example, an In—Ga—Zn-based oxide with an atomic ratio ofIn:Ga:Zn=1:1:1 (=1/3:1/3:1/3), In:Ga:Zn=2:2:1 (=2/5:2/5:1/5), orIn:Ga:Zn=3:1:2 (=1/2:1/6:1/3), or any of oxides whose composition is inthe neighborhood of the above compositions can be used. Alternatively,an In—Sn—Zn-based oxide with an atomic ratio of In:Sn:Zn=1:1:1(=1/3:1/3:1/3), In:Sn:Zn=2:1:3 (=1/3:1/6:1/2), or In:Sn:Zn=2:1:5(=1/4:1/8:5/8), or any of oxides whose composition is in theneighborhood of the above compositions may be used.

However, the composition is not limited to those described above, and amaterial having an appropriate composition may be used depending onneeded semiconductor characteristics (such as mobility, thresholdvoltage, and variation). In order to obtain needed semiconductorcharacteristics, it is preferable that the carrier concentration, theimpurity concentration, the defect density, the atomic ratio of a metalelement to oxygen, the interatomic distance, the density, and the likebe set to appropriate values.

For example, it is relatively easy to obtain high mobility with anIn—Sn—Zn-based oxide. However, it is possible to obtain high mobilityalso with an In—Ga—Zn-based oxide by reducing the defect density in abulk.

Note that for example, the expression “the composition of an oxidecontaining In, Ga, and Zn at the atomic ratio, In:Ga:Zn=a:b:c (a+b+c=1),is in the neighborhood of the composition of an oxide containing In, Ga,and Zn at the atomic ratio, In:Ga:Zn=A:B:C (A+B+C=1)” means that a, b,and c satisfy the following relation: (a−A)²+(b−B)²+(c−C)²≦r², and r maybe 0.05, for example. The same applies to other oxides.

It is preferable that a high-purity gas from which impurities such ashydrogen, water, a hydroxyl group, and a hydride are removed be used asa sputtering gas used for the formation of the oxide semiconductor film.

For the oxide semiconductor layer 120 in the initial stage, a filmhaving a single crystal state, a polycrystalline (also referred to aspolycrystal) state, an amorphous state, or the like can be used. Theoxide semiconductor layer is preferably a c-axis aligned crystallineoxide semiconductor (CAAC-OS) film.

Sputtering may be performed to form an oxide semiconductor film of aCAAC-OS film. In order to obtain a CAAC-OS film by sputtering, it isimportant to form hexagonal crystals in an initial stage of depositionof an oxide semiconductor film and cause crystal growth from thehexagonal crystals as seeds. In order to achieve this, it is preferablethat the distance between the target and the substrate be made longer(e.g., 150 mm to 200 mm) and the substrate heating temperature be 100°C. to 500° C., more preferably 200° C. to 400° C., still preferably 250°C. to 300° C. In addition to this, the deposited oxide semiconductorfilm is subjected to heat treatment at a temperature higher than thesubstrate heating temperature in the deposition, so that micro-defectsin the film and defects at the interface of a stacked layer can becompensated.

The CAAC-OS film is not completely single crystal nor completelyamorphous. The CAAC-OS film is an oxide semiconductor film with acrystal-amorphous mixed phase structure where crystal parts andamorphous parts are included in an amorphous phase. Note that in mostcases, the crystal part fits inside a cube whose one side is less than100 nm. From an observation image obtained with a transmission electronmicroscope (TEM), a boundary between an amorphous part and a crystalpart in the CAAC-OS film is not clear. Further, with the TEM, a grainboundary in the CAAC-OS film is not found. Thus, in the CAAC-OS film, areduction in electron mobility, due to the grain boundary, issuppressed.

In each of the crystal parts included in the CAAC-OS film, a c-axis isaligned in a direction parallel to a normal vector of a surface wherethe CAAC-OS film is formed or a normal vector of a surface of theCAAC-OS film, triangular or hexagonal atomic arrangement which is seenfrom the direction perpendicular to the a-b plane is formed, and metalatoms are arranged in a layered manner or metal atoms and oxygen atomsare arranged in a layered manner when seen from the directionperpendicular to the c-axis. Note that, among crystal parts, thedirections of the a-axis and the b-axis of one crystal part may bedifferent from those of another crystal part. In this specification, asimple term “perpendicular” includes a range from 85° to 95°. Inaddition, a simple term “parallel” includes a range from −5° to 5°.

In the CAAC-OS film, distribution of crystal parts is not necessarilyuniform. For example, in the formation process of the CAAC-OS film, inthe case where crystal growth occurs from a surface side of the oxidesemiconductor film, the proportion of crystal parts in the vicinity ofthe surface of the oxide semiconductor film is higher than that in thevicinity of the surface where the oxide semiconductor film is formed insome cases. Further, when an impurity is added to the CAAC-OS film, thecrystal part in a region to which the impurity is added becomesamorphous in some cases.

Since the c-axes of the crystal parts included in the CAAC-OS film arealigned in the direction parallel to a normal vector of a surface wherethe CAAC-OS film is formed or a normal vector of a surface of theCAAC-OS film, the directions of the c-axes may be different from eachother depending on the shape of the CAAC-OS film (the cross-sectionalshape of the surface where the CAAC-OS film is formed or thecross-sectional shape of the surface of the CAAC-OS film). Note that thedirection of c-axis of the crystal part is the direction parallel to anormal vector of the surface where the CAAC-OS film is formed just afterthe formation of the CAAC-OS film or a normal vector of the surface ofthe CAAC-OS film just after the formation of the CAAC-OS film. Thecrystal part is formed by the film formation or by performing treatmentfor crystallization such as heat treatment after the film formation.

With the use of the CAAC-OS film in a transistor, a change in electriccharacteristics of the transistor due to irradiation with visible lightor ultraviolet light is small. Thus, the transistor has highreliability.

For the deposition of the CAAC-OS film, the following conditions arepreferably used.

By reducing the amount of impurities entering the CAAC-OS film duringthe deposition, the crystal state can be prevented from being broken bythe impurities. For example, impurities (e.g., hydrogen, water, carbondioxide, or nitrogen) which exist in the deposition chamber may bereduced. Furthermore, impurities in a deposition gas may be reduced.Specifically, a deposition gas whose dew point is −80° C. or lower,preferably −100° C. or lower is used.

By increasing the substrate heating temperature during the deposition,migration of a sputtered particle is likely to occur after the sputteredparticle reaches a substrate surface. Specifically, the substrateheating temperature during the deposition is higher than or equal to100° C. and lower than or equal to 740° C., preferably higher than orequal to 200° C. and lower than or equal to 500° C. By increasing thesubstrate heating temperature during the deposition, when theflat-plate-like sputtered particle reaches the substrate, migrationoccurs on the substrate surface, so that a flat plane of theflat-plate-like sputtered particle is attached to the substrate.

Furthermore, it is preferable that the proportion of oxygen in thedeposition gas be increased and the power be optimized in order toreduce plasma damage at the deposition. The proportion of oxygen in thedeposition gas is 30 vol % or higher, preferably 100 vol %.

Note that the oxide semiconductor layer 120 may have a structure inwhich a plurality of oxide semiconductor layers is stacked. For example,the oxide semiconductor layer 120 may be a stack of a first oxidesemiconductor layer and a second oxide semiconductor layer that areformed using metal oxides with different compositions.

Further, the constituent elements of the first oxide semiconductor layerand the second oxide semiconductor layer may be the same but thecompositions of the constituent elements of the first oxidesemiconductor layer and the second oxide semiconductor layer may bedifferent from each other. For example, the first oxide semiconductorlayer may have an atomic ratio of In:Ga:Zn=1:1:1, and the second oxidesemiconductor layer may have an atomic ratio of In:Ga:Zn=3:1:2.Alternatively, the first oxide semiconductor layer may have an atomicratio of In:Ga:Zn=1:3:2, and the second oxide semiconductor layer mayhave an atomic ratio of In:Ga:Zn=2:1:3.

In this case, one of the first oxide semiconductor layer and the secondoxide semiconductor layer which is closer to the gate electrode (on achannel side) preferably contains In and Ga such that their contentssatisfy In>Ga. The other which is farther from the gate electrode (on aback channel side) preferably contains In and Ga such that theircontents satisfy In≦Ga (the Ga content is equal to or in excess of theIn content).

In an oxide semiconductor, the s orbital of heavy metal mainlycontributes to carrier transfer, and overlap of the s orbitals is likelyto increase when the In content in the oxide semiconductor is increased.Therefore, an oxide having a composition of In>Ga has higher mobilitythan an oxide having a composition of In≦Ga. Further, the formationenergy of oxygen vacancy is larger and thus oxygen vacancy is lesslikely to occur in Ga than in In; thus, the oxide having a compositionof In≦Ga has more stable characteristics than the oxide having acomposition of In>Ga.

An oxide semiconductor having a composition of In>Ga is used on thechannel side and an oxide semiconductor having a composition of In≦Ga isused on the back channel side, whereby the mobility and reliability ofthe transistor can be further improved.

Further, oxide semiconductors having different crystallinities may beused for the first oxide semiconductor layer and the second oxidesemiconductor layer. That is, the first oxide semiconductor layer andthe second oxide semiconductor layer may be formed by using acombination of any of a single crystal oxide semiconductor film, apolycrystalline oxide semiconductor film, an amorphous oxidesemiconductor film, and a CAAC-OS film as appropriate. When an amorphousoxide semiconductor film is used for at least one of the first oxidesemiconductor layer and the second oxide semiconductor layer, internalstress or external stress of the oxide semiconductor layer 120 isrelieved, variation in characteristics of a transistor is reduced, andthe reliability of the transistor can be further improved.

On the other hand, an amorphous oxide semiconductor film is likely toabsorb an impurity which serves as a donor, such as hydrogen, and islikely to generate oxygen vacancy, and thus easily becomes n-type. Thus,the oxide semiconductor layer on the channel side is preferably formedusing a crystalline oxide semiconductor such as a CAAC-OS.

Further, the oxide semiconductor layer 120 may have a layered structureof three or more layers where an amorphous oxide semiconductor layer issandwiched between a plurality of crystalline oxide semiconductorlayers. Moreover, the oxide semiconductor layer 120 may have a structurein which a crystalline oxide semiconductor layer and an amorphous oxidesemiconductor layer are alternately stacked.

The above structures used when the oxide semiconductor layer 120 has alayered structure of a plurality of layers can be employed incombination as appropriate.

Note that in the case where the oxide semiconductor layer 120 has alayered structure of a plurality of layers, oxygen may be introducedeach time the oxide semiconductor layer is formed. Oxygen may beintroduced by heat treatment in an oxygen atmosphere, an ionimplantation method, an ion doping method, a plasma immersion ionimplantation method, plasma treatment in an atmosphere containingoxygen, or the like.

Oxygen is introduced each time the oxide semiconductor layer is formed,whereby the effect of reducing oxygen vacancies in the oxidesemiconductor can be improved.

In this embodiment, an example of using a CAAC-OS film as the oxidesemiconductor layer 120 in the initial stage is described.

Then, the gate insulating film 130 is formed over the oxidesemiconductor layer 120 by a plasma CVD method, a sputtering method, orthe like. As the gate insulating film 130, a silicon oxide film, agallium oxide film, an aluminum oxide film, a silicon nitride film, asilicon oxynitride film, an aluminum oxynitride film, or a siliconnitride oxide film can be used.

When the gate insulating film 130 is formed using a high-k material suchas hafnium oxide, yttrium oxide, hafnium silicate (HfSi_(x)O_(y) (x>0,y>0)), hafnium silicate to which nitrogen is added, hafnium aluminate(HfAl_(x)O_(y) (x>0, y>0)), or lanthanum oxide, gate leakage current canbe reduced. The structure of the gate insulating film 130 is not limitedto a single-layer structure of any of the above materials, and may be alayered structure thereof.

Note that the gate insulating film 130 preferably contains oxygen andcontains as little impurities such as water and hydrogen as possiblebecause it is an insulating film in contact with the oxide semiconductorlayer 120. In the case of using a plasma CVD method, hydrogen iscontained in a source gas; thus, it is difficult to reduce the hydrogenconcentration in the film as compared to the case of using a sputteringmethod. For that reason, in the case where the gate insulating film 130is formed by a plasma CVD method, it is preferable to perform heattreatment for reducing or removing hydrogen (dehydration ordehydrogenation treatment) after the deposition.

The heat treatment is performed at a temperature higher than or equal to250° C. and lower than or equal to 650° C., preferably higher than orequal to 450° C. and lower than or equal to 600° C. Note that in thecase where a glass substrate is used, the heat treatment is performed ata temperature lower than the strain point of the glass substrate. Forexample, the substrate is introduced into an electric furnace, which isone kind of heat treatment apparatus, and heat treatment is performed onthe gate insulating film 130 at 650° C. for one hour in a vacuum(reduced pressure) atmosphere.

Note that the heat treatment apparatus is not limited to an electricfurnace, and a device for heating an object to be processed by heatconduction or heat radiation from a heating element such as a resistanceheating element may be alternatively used. For example, a rapid thermalanneal (RTA) apparatus such as a gas rapid thermal anneal (GRTA)apparatus or a lamp rapid thermal anneal (LRTA) apparatus can be used.An LRTA apparatus is an apparatus for heating an object to be processedby radiation of light (an electromagnetic wave) emitted from a lamp suchas a halogen lamp, a metal halide lamp, a xenon arc lamp, a carbon arclamp, a high pressure sodium lamp, or a high pressure mercury lamp. AGRTA apparatus is an apparatus for performing heat treatment using ahigh-temperature gas. As the high-temperature gas, an inert gas whichdoes not react with an object to be processed by heat treatment, such asnitrogen or a rare gas such as argon is used. Note that in the casewhere a GRTA apparatus is used as the heat treatment apparatus, thesubstrate may be heated in an inert gas heated to a high temperature of650° C. to 700° C. because the heat treatment time is short.

The heat treatment may be performed in an atmosphere of nitrogen,oxygen, ultra-dry air (air in which the water content is 20 ppm or less,preferably 1 ppm or less, further preferably 10 ppb or less), or a raregas (argon, helium, or the like). Note that it is preferable that water,hydrogen, or the like be not contained in the atmosphere of nitrogen,oxygen, ultra-dry air, or a rare gas. The purity of nitrogen, oxygen, ora rare gas which is introduced into a heat treatment apparatus ispreferably 6N (99.9999%) or higher, more preferably 7N (99.99999%) orhigher (that is, the impurity concentration is preferably 1 ppm orlower, more preferably 0.1 ppm or lower).

Through the heat treatment, the gate insulating film 130 can bedehydrated or dehydrogenated, whereby the gate insulating film 130 fromwhich impurities such as hydrogen and water, which cause a change incharacteristics of a transistor, are removed can be formed.

In heating treatment where dehydration or dehydrogenation treatment isperformed, it is preferable that a surface of the gate insulating film130 be not in a state where hydrogen, moisture, or the like is preventedfrom being released (for example, by providing a film or the like whichis not permeable to hydrogen, moisture, or the like), but in a statewhere the surface of the gate insulating film 130 is exposed.

The heat treatment for dehydration or dehydrogenation may be performedmore than once, and may also serve as another heat treatment.

Oxygen adding treatment may be performed on the dehydrated ordehydrogenated gate insulating film 130. Oxygen can be supplied to thegate insulating film 130 by an ion implantation method, an ion dopingmethod, a plasma immersion ion implantation method, or a plasmatreatment method, for example. Through this treatment, oxygen may besupplied also to the oxide semiconductor layer 120.

Next, a conductive film is formed over the gate insulating film 130 by asputtering method or the like, and then processed into the gateelectrode 140 by a photolithography method and an etching method (seeFIG. 4B).

The conductive film to be the gate electrode 140 can be formed using ametal material such as molybdenum, titanium, tantalum, tungsten,aluminum, copper, chromium, neodymium, or scandium or an alloy materialwhich contains any of these materials as its main component. Asemiconductor film which is doped with an impurity element such asphosphorus and is typified by a polycrystalline silicon film, or asilicide film of nickel silicide or the like can also be used as thegate electrode 140. The gate electrode 140 has either a single-layerstructure or a layered structure.

The gate electrode 140 can also be formed using a conductive materialsuch as indium oxide-tin oxide, indium oxide containing tungsten oxide,indium zinc oxide containing tungsten oxide, indium oxide containingtitanium oxide, indium tin oxide containing titanium oxide, indiumoxide-zinc oxide, or indium tin oxide to which silicon oxide is added.Alternatively, the gate electrode 140 may have a layered structure ofthe above conductive material and the above metal material.

As the gate electrode 140, a metal oxide containing nitrogen,specifically, an In—Ga—Zn—O film containing nitrogen, an In—Sn—O filmcontaining nitrogen, an In—Ga—O film containing nitrogen, an In—Zn—Ofilm containing nitrogen, a Sn—O film containing nitrogen, an In—O filmcontaining nitrogen, or a metal nitride (e.g., InN or SnN) film can beused. Such a film has a work function of 5 eV (electron volts) orhigher, preferably 5.5 eV or higher, and use of this film as the gateelectrode enables the threshold voltage of a transistor to be a positivevalue. Accordingly, a so-called normally-off switching element can beobtained.

For example, the gate electrode 140 can be formed using a conductivelayer having a three-layer structure in which tungsten nitride thatprevents diffusion of copper is used for one of an upper layer and alower layer, tantalum nitride is used for the other, and copper is usedfor a medium layer. In the case where such an electrode structure isemployed, a photolithography process and an etching step need to beadditionally performed to confine copper; however, the electrodestructure has a significantly high effect of preventing diffusion ofcopper, so that the reliability of the transistor can be improved.

End portions of the gate electrode 140 and an electrode or a wiring thatcan be formed through the same steps as the gate electrode 140preferably have tapered shapes. When the end portion of the electrode orthe wiring is tapered, coverage with an insulating film or the likeformed thereover can be improved, which prevents a reduction inelectrical characteristics and a reduction in reliability that arecaused when the coverage is poor. Note that the taper angle of the endportion of the electrode or the wiring is preferably 40° to 80°.

Heat treatment may be performed after the gate electrode 140 is formed.For example, the heat treatment may be performed with a GRTA apparatusat 650° C. for 1 minute to 5 minutes. Alternatively, the heat treatmentmay be performed with an electric furnace at 500° C. for 30 minutes to 1hour.

Next, oxygen 101 is added to the oxide semiconductor layer 120 with thegate electrode 140 used as a mask, whereby the channel formation region120 a and the source region or drain region 120 b are formed (see FIG.4C). For example, oxygen can be supplied to the oxide semiconductorlayer 120 through the gate insulating film 130 by an ion implantationmethod, an ion doping method, a plasma immersion ion implantationmethod, a plasma treatment method, or the like. Through this treatment,oxygen may be supplied also to the gate insulating film 130.

In this embodiment, the oxygen 101 is implanted into a region to be thesource region or drain region 120 b by an ion implantation method. Forexample, in the case where the oxide semiconductor layer 120 has athickness of 30 nm and the gate insulating film 130 has a thickness of20 nm, O⁺ with a dose of 1×10¹⁵/cm² to 5×10¹⁶/cm² may be implanted at anacceleration voltage of 5 kV to 30 kV. Alternatively, O₂ ⁺ with a doseof 5×10¹⁴/cm² to 2.5×10¹⁶/cm² may be implanted at an accelerationvoltage of 10 kV to 60 kV.

Here, the oxide semiconductor layer 120 in the initial stage is aCAAC-OS film, and the channel formation region maintains its state;however, the orderliness of atoms forming a crystal component in theregion to be the source region or drain region 120 b is disrupted bydamage due to implanted oxygen atoms, and the region becomes amorphous.

Next, in the oxide semiconductor layer 120, an impurity is added to theregion to be the source region or drain region 120 b in order that theresistance of the region is lowered, whereby the source region or drainregion 120 b is formed. As a method for adding the impurity, an ionimplantation method, an ion doping method, a plasma immersion ionimplantation method, or the like can be used.

As the impurity for improving the conductivity of the oxidesemiconductor layer 120, for example, one or more selected from thefollowing can be used: phosphorus (P), arsenic (As), antimony (Sb),boron (B), aluminum (Al), nitrogen (N), argon (Ar), helium (He), neon(Ne), indium (In), fluorine (F), chlorine (Cl), titanium (Ti), zinc(Zn), and carbon (C).

The addition of the impurity may be controlled by setting the additionconditions such as the acceleration voltage and the dose, or thethickness of the film through which the dopant passes as appropriate.For example, in the case where phosphorus is used as the impurity addedto the region to be the source region or drain region 120 b, theimpurity concentration in the region to which the impurity is added ispreferably higher than or equal to 5×10¹⁸/cm³ and lower than or equal to1×10²²/cm³.

Note that CO⁺ or CO₂ ⁺ is implanted through the ion implantation step,whereby oxygen and carbon can be added to the source region or drainregion 120 b concurrently; thus, the number of times of the ionimplantation step can be one. Since CO₂ ⁺ has larger mass than O₂ ⁺, thepeak position of the implantation profile can be in a shallow region;thus, CO₂ ⁺ is more suitable for implantation into a thin film.

Note that the addition of the impurity may be performed in the statewhere the substrate is heated. The addition of the impurity to the oxidesemiconductor layer 120 may be performed a plurality of times, and aplurality of kinds of impurity may be used. The addition of the impuritymay be performed before the step of adding oxygen to the region to bethe source region or drain region 120 b, after a step of forming theprotective film 160, or after a step of performing heat treatment on theoxide semiconductor layer 120.

Then, the protective film 160 is preferably formed over the gateinsulating film 130 and the gate electrode 140. As the protective film160, an insulating film such as a silicon oxide film, a gallium oxidefilm, an aluminum oxide film, a silicon nitride film, a siliconoxynitride film, an aluminum oxynitride film, or a silicon nitride oxidefilm can be used.

As the protective film 160, an aluminum oxide film is particularlypreferable. The aluminum oxide film has a high blocking effect ofpreventing penetration of both oxygen and an impurity such as hydrogenor moisture.

The aluminum oxide film can be directly formed by a sputtering method orthe like. Alternatively, the aluminum oxide film can be formed in such amanner that an aluminum (Al) film is formed by a sputtering method orthe like and then oxygen plasma treatment, oxygen ion implantation,oxygen ion doping, or the like is performed.

The protective film 160 may have a layered structure of an aluminumoxide film and any one or more of a silicon oxide film, a gallium oxidefilm, a silicon nitride film, a silicon oxynitride film, an aluminumoxynitride film, and a silicon nitride oxide film.

The protective film 160 may be subjected to oxygen adding treatment. Forexample, oxygen can be supplied to the protective film 160 by an ionimplantation method, an ion doping method, a plasma immersion ionimplantation method, a plasma treatment method, or the like.

Then, the oxide semiconductor layer 120 is subjected to heat treatment,whereby oxygen added to the source region or drain region 120 b is madeto be actively diffused in the lateral direction and oxygen vacanciesformed in the channel formation region 120 a through the above heattreatment for the dehydration or dehydrogenation of the gate insulatingfilm 130 are filled. The heat treatment for the oxide semiconductorlayer 120 can be performed similarly to the above heat treatment for thedehydration or dehydrogenation of the gate insulating film 130. Althoughthis heat treatment may be performed anytime after the step of addingoxygen to the region to be the source region or drain region 120 b, itis preferably performed after the formation of the protective film 160.The protective film 160 prevents release of oxygen through theprotective film 160; thus, oxygen added to the source region or drainregion 120 b can be efficiently diffused to the channel formation region120 a.

Oxygen added to the source region or drain region 120 b is partlydiffused to the channel formation region 120 a, and the amount of oxygenin the source region or drain region 120 b is kept larger than that inthe channel formation region 120 a. Therefore, the source region ordrain region 120 b can serve as a source of oxygen for the channelformation region 120 a continuously, and oxygen vacancies in the channelformation region 120 a which are caused owing to long-time operation oroperation environment of the transistor can be filled. Note thatexcessive oxygen in the source region or drain region 120 b can bediffused to the channel formation region 120 a even at room temperature.

Then, the planarization film 170 is formed over the protective film 160as needed. For the planarization film, an organic material having heatresistance such as a polyimide-based resin, an acrylic-based resin, apolyimide amide-based resin, a benzocyclobutene-based resin, apolyamide-based resin, or an epoxy-based resin can be used as well asthe insulating film that can be used as the protective film 160. Inaddition to such organic materials, it is also possible to use alow-dielectric constant material (low-k material), a siloxane-basedresin, phosphosilicate glass (PSG), borophosphosilicate glass (BPSG), orthe like. Note that the planarization film may be formed by stacking aplurality of insulating films formed using any of these materials.Alternatively, a surface of the formed film may be planarized by a CMPmethod or the like.

For example, a 1500-nm-thick acrylic resin film may be formed as theplanarization film 170. The acrylic resin film can be formed in such amanner that an acrylic resin is applied by a coating method and thenbaked (e.g., at 250° C. for one hour in a nitrogen atmosphere).

Next, contact holes are formed in the planarization film 170, theprotective film 160, and the gate insulating film 130 by aphotolithography method and an etching method. Then, a conductive filmis formed over the planarization film 170 by a sputtering method or thelike so as to fill the contact holes, and is then processed into thesource electrode 150 a and the drain electrode 150 b by aphotolithography method and an etching method (see FIG. 4D).

The conductive film can be formed using an element selected fromaluminum, chromium, copper, tantalum, titanium, molybdenum, tungsten,and the like; an alloy containing any of these elements as a component;an alloy containing any of these elements in combination; or the like.Further, the conductive film may have a single-layer structure or alayered structure of two or more layers. For example, a structure may beemployed in which a film of a refractory metal such as chromium,tantalum, titanium, molybdenum, or tungsten or a conductive nitride filmthereof is stacked over and/or below a metal film of aluminum, copper,or the like. Further, one or more materials selected from manganese,magnesium, zirconium, beryllium, neodymium, and scandium may be used.

Through the above steps, the transistor 191 illustrated in FIGS. 1A and1B can be manufactured.

A method for manufacturing the transistor 192 illustrated in FIGS. 2Aand 2B is as follows.

First, the oxide semiconductor layer 120 is formed over the baseinsulating film 110 over the substrate 100 (see FIG. 5A).

Next, the source electrode 150 a and the drain electrode 150 b areformed in contact with part of the oxide semiconductor layer, and thenthe gate insulating film 130 is formed over the oxide semiconductorlayer, and the source electrode and the drain electrode (see FIG. 5B).

Then, oxygen and an impurity for improving conductivity are added to theoxide semiconductor layer 120 by an ion implantation method or the likewith the gate electrode 140, the source electrode 150 a, and the drainelectrode 150 b as masks, whereby the channel formation region 120 a,the source region or drain region 120 b, and the region 120 c are formed(see FIG. 5C).

Next, heat treatment is performed after the protective film 160 isformed, whereby oxygen is diffused from the source region or drainregion 120 b to the channel formation region 120 a.

Then, the planarization film 170 is formed as needed (see FIG. 5D).

Through the above steps, the transistor 192 illustrated in FIGS. 2A and2B can be manufactured. Note that the transistor 192 can be formed usingmaterials similar to those of the transistor 191, and the method formanufacturing the transistor 191 can be referred to for the detailsthereof.

A method for manufacturing the transistor 193 illustrated in FIGS. 3Aand 3B is as follows.

First, the source electrode 150 a and the drain electrode 150 b areformed over the base insulating film 110 over the substrate 100.

Then, the oxide semiconductor layer 120 is formed in contact with partof the source electrode 150 a and the drain electrode 150 b (see FIG.6A).

Next, the gate insulating film 130 is formed over the source electrode150 a, the drain electrode 150 b, and the oxide semiconductor layer 120,and then the gate electrode 140 is formed over the gate insulating film(see FIG. 6B).

Next, oxygen is added to the oxide semiconductor layer 120 by an ionimplantation method or the like with the gate electrode 140 used as amask, whereby the channel formation region 120 a and the source regionor drain region 120 b are formed (see FIG. 6C). Note that an impurityfor improving conductivity may be added to the source region or drainregion 120 b.

Next, heat treatment is performed after the protective film 160 isformed, whereby oxygen is diffused from the source region or drainregion 120 b to the channel formation region 120 a.

Then, the planarization film 170 is formed as needed (see FIG. 6D).

Through the above steps, the transistor 193 illustrated in FIGS. 3A and3B can be manufactured. Note that the transistor 193 can be formed usingmaterials similar to those of the transistor 191, and the method formanufacturing the transistor 191 can be referred to for the detailsthereof.

This embodiment can be implemented in appropriate combination with anyof the structures described in the other embodiments.

Embodiment 2

In this embodiment, an example of a semiconductor device (memory device)which includes a transistor according to one embodiment of the presentinvention, which can hold stored data even when not powered, and whichhas an unlimited number of write cycles will be described with referenceto drawings.

FIGS. 7A and 7B illustrate an example of a configuration of thesemiconductor device. FIG. 7A is a cross-sectional view of thesemiconductor device, and FIG. 7B is a circuit diagram of thesemiconductor device.

The semiconductor device illustrated in FIGS. 7A and 7B includes atransistor 3200 including a first semiconductor material in a lowerportion, and a transistor 3202 including a second semiconductor materialand a capacitor 3204 in an upper portion. An example of applying thetransistor illustrated in FIGS. 1A and 1B in Embodiment 1 to thetransistor 3202 is described. One electrode of the capacitor 3204 isformed using the same material as a gate electrode of the transistor3202, the other electrode thereof is formed using the same material as asource electrode and a drain electrode of the transistor 3202, and adielectric thereof is formed using the same material as a protectivefilm and a planarization film of the transistor 3202; thus, thecapacitor 3204 can be formed concurrently with the transistor 3202.

Here, the first semiconductor material and the second semiconductormaterial are preferably materials having different band gaps. Forexample, the first semiconductor material may be a semiconductormaterial (such as silicon) other than an oxide semiconductor, and thesecond semiconductor material may be an oxide semiconductor. Atransistor including a material other than an oxide semiconductor canoperate at high speed easily. On the other hand, a transistor includingan oxide semiconductor enables charge to be held for a long time owingto its characteristics.

Although both of the above transistors are n-channel transistors in thefollowing description, it is needless to say that p-channel transistorscan be used. The specific structure of the semiconductor device, such asthe material used for the semiconductor device and the structure of thesemiconductor device, is not necessarily limited to those described hereexcept for the use of the transistor described in Embodiment 1, which isformed using an oxide semiconductor for holding data.

The transistor 3200 in FIG. 7A includes a channel formation regionprovided in a substrate 3000 including a semiconductor material (such ascrystalline silicon), impurity regions provided such that the channelformation region is sandwiched therebetween, intermetallic compoundregions provided in contact with the impurity regions, a gate insulatingfilm provided over the channel formation region, and a gate electrodeprovided over the gate insulating film. Note that a transistor whosesource electrode and drain electrode are not illustrated in a drawingmay also be referred to as a transistor for the sake of convenience.Further, in such a case, in description of a connection of a transistor,a source region and a source electrode may be collectively referred toas a source electrode, and a drain region and a drain electrode may becollectively referred to as a drain electrode. That is, in thisspecification, the term “source electrode” may include a source region.

Further, an element isolation insulating layer 3106 is formed on thesubstrate 3000 so as to surround the transistor 3200, and an insulatinglayer 3220 is formed so as to cover the transistor 3200. Note that theelement isolation insulating layer 3106 can be formed by an elementisolation technique such as local oxidation of silicon (LOCOS) orshallow trench isolation (STI).

For example, the transistor 3200 formed using a crystalline siliconsubstrate can operate at high speed. Thus, when the transistor is usedas a reading transistor, data can be read at high speed. As treatmentprior to formation of the transistor 3202 and the capacitor 3204, CMPtreatment is performed on the insulating layer 3220 covering thetransistor 3200, whereby the insulating layer 3220 is planarized and, atthe same time, an upper surface of the gate electrode of the transistor3200 is exposed.

A connection wiring 3210 is provided over the gate electrode of thetransistor 3200 to be electrically connected to the gate electrode. Thetransistor 3202 is provided over the insulating layer 3220, and one ofthe source electrode and the drain electrode of the transistor 3202 iselectrically connected to the connection wiring 3210. Note that theconnection wiring 3210 also serves as the one electrode of the capacitor3204.

The transistor 3202 in FIG. 7A is a bottom-gate transistor in which achannel is formed in an oxide semiconductor layer. Since the off-statecurrent of the transistor 3202 is small, stored data can be held for along time owing to such a transistor. In other words, refresh operationbecomes unnecessary or the frequency of the refresh operation in asemiconductor memory device can be extremely lowered, which leads to asufficient reduction in power consumption.

The transistor 3200 and the capacitor 3204 can be formed so as tooverlap with each other as illustrated in FIG. 7A, whereby the areaoccupied by them can be reduced. Accordingly, the degree of integrationof the semiconductor device can be increased.

An example of a circuit configuration corresponding to FIG. 7A isillustrated in FIG. 7B.

In FIG. 7B, a first wiring (1st Line) is electrically connected to asource electrode of the transistor 3200. A second wiring (2nd Line) iselectrically connected to a drain electrode of the transistor 3200. Athird wiring (3rd Line) is electrically connected to one of the sourceand drain electrodes of the transistor 3202, and a fourth wiring (4thLine) is electrically connected to the gate electrode of the transistor3202. The gate electrode of the transistor 3200 and the other of thesource and drain electrodes of the transistor 3202 are electricallyconnected to the one electrode of the capacitor 3204. A fifth wiring(5th Line) is electrically connected to the other electrode of thecapacitor 3204.

The semiconductor device in FIG. 7B utilizes a characteristic in whichthe potential of the gate electrode of the transistor 3200 can be held,and thus enables data writing, holding, and reading as follows.

Writing and holding of data will be described. First, the potential ofthe fourth wiring is set to a potential at which the transistor 3202 isturned on, so that the transistor 3202 is turned on. Accordingly, thepotential of the third wiring is supplied to the gate electrode of thetransistor 3200 and to the capacitor 3204. That is, predetermined chargeis supplied to the gate electrode of the transistor 3200 (writing).Here, one of two kinds of charges providing different potential levels(hereinafter referred to as a low-level charge and a high-level charge)is supplied. After that, the potential of the fourth wiring is set to apotential at which the transistor 3202 is turned off, so that thetransistor 3202 is turned off. Thus, the charge supplied to the gateelectrode of the transistor 3200 is held (holding).

Since the off-state current of the transistor 3202 is significantlysmall, the charge of the gate electrode of the transistor 3200 is heldfor a long time.

Next, reading of data will be described. By supplying an appropriatepotential (a reading potential) to the fifth wiring while supplying apredetermined potential (a constant potential) to the first wiring, thepotential of the second wiring varies depending on the amount of chargeheld at the gate electrode of the transistor 3200. This is because ingeneral, when the transistor 3200 is an n-channel transistor, anapparent threshold voltage V_(th) _(—) _(H) in the case where thehigh-level charge is given to the gate electrode of the transistor 3200is lower than an apparent threshold voltage V_(th) _(—) _(L) in the casewhere the low-level charge is given to the gate electrode of thetransistor 3200. Here, an apparent threshold voltage refers to thepotential of the fifth wiring which is needed to turn on the transistor3200. Thus, the potential of the fifth wiring is set to a potentialV_(o) which is between V_(th) _(—) _(H) and V_(th) _(—) _(L), wherebycharge supplied to the gate electrode of the transistor 3200 can bedetermined. For example, in the case where the high-level charge issupplied in writing, when the potential of the fifth wiring is V₀(>V_(th) _(—) _(H)), the transistor 3200 is turned on. In the case wherethe low-level charge is supplied in writing, even when the potential ofthe fifth wiring is V₀ (<V_(th) _(—) _(L)), the transistor 3200 remainsoff. Therefore, the data held can be read by determining the potentialof the second wiring.

Note that in the case where memory cells are arrayed, it is necessarythat data of only a desired memory cell can be read. The fifth wiringsof memory cells from which data is not read may be supplied with apotential at which the transistor 3200 is turned off regardless of thestate of the gate electrode, that is, a potential lower than V_(th) _(—)_(H). Alternatively, the fifth wirings may be supplied with a potentialat which the transistor 3200 is turned on regardless of the state of thegate electrode, that is, a potential higher than V_(th) _(—) _(L).

When including a transistor having a channel formation region formedusing an oxide semiconductor and having extremely small off-statecurrent, the semiconductor device described in this embodiment can storedata for an extremely long period. In other words, refresh operationbecomes unnecessary or the frequency of the refresh operation can beextremely lowered, which leads to a sufficient reduction in powerconsumption. Moreover, stored data can be held for a long period evenwhen power is not supplied (note that a potential is preferably fixed).

Further, in the semiconductor device described in this embodiment, highvoltage is not needed for writing data and there is no problem ofdeterioration of elements. For example, unlike a conventionalnonvolatile memory, it is not necessary to inject and extract electronsinto and from a floating gate, and thus a problem such as deteriorationof a gate insulating film does not arise at all. That is, thesemiconductor device according to the disclosed invention does not havea limitation on the number of times data can be rewritten, which is aproblem of a conventional nonvolatile memory, and the reliabilitythereof is drastically improved. Furthermore, data is written dependingon the on state and the off state of the transistor, whereby high-speedoperation can be easily achieved.

As described above, a miniaturized and highly integrated semiconductordevice having favorable electrical characteristics and a method formanufacturing the semiconductor device can be provided.

This embodiment can be implemented in appropriate combination with anyof the structures described in the other embodiments.

Embodiment 3

In this embodiment, a semiconductor device including the transistordescribed in Embodiment 1, which can hold stored data even when notpowered, which does not have a limitation on the number of write cycles,and which has a structure different from the structure described inEmbodiment 2 will be described.

FIG. 8A illustrates an example of a circuit configuration of asemiconductor device, and FIG. 8B is a conceptual diagram illustratingan example of a semiconductor device. As a transistor 4162 included inthe semiconductor device, the transistor described in Embodiment 1 canbe used. A capacitor 4254 can be formed similarly to the capacitor 3204described in Embodiment 2 through the same process as and concurrentlywith the transistor 4162.

In the semiconductor device illustrated in FIG. 8A, a bit line BL iselectrically connected to a source electrode of the transistor 4162, aword line WL is electrically connected to a gate electrode of thetransistor 4162, and a drain electrode of the transistor 4162 iselectrically connected to a first terminal of the capacitor 4254.

Next, writing and holding of data in the semiconductor device (a memorycell 4250) illustrated in FIG. 8A will be described.

First, the potential of the word line WL is set to a potential at whichthe transistor 4162 is turned on, and the transistor 4162 is turned on.Accordingly, the potential of the bit line BL is supplied to the firstterminal of the capacitor 4254 (writing). After that, the potential ofthe word line WL is set to a potential at which the transistor 4162 isturned off, so that the transistor 4162 is turned off. Thus, thepotential at the first terminal of the capacitor 4254 is held (holding).

In addition, the amount of off-state current is extremely small in thetransistor 4162 which uses an oxide semiconductor. For that reason, thepotential of the first terminal of the capacitor 4254 (or a chargeaccumulated in the capacitor 4254) can be held for an extremely longperiod by turning off the transistor 4162.

Next, reading of data will be described. When the transistor 4162 isturned on, the bit line BL which is in a floating state and thecapacitor 4254 are electrically connected to each other, and the chargeis redistributed between the bit line BL and the capacitor 4254. As aresult, the potential of the bit line BL is changed. The amount ofchange in potential of the bit line BL varies depending on the potentialof the first terminal of the capacitor 4254 (or the charge accumulatedin the capacitor 4254).

For example, the potential of the bit line BL after chargeredistribution is (C_(B)×V_(B0)+C×V)/(C_(B)+C), where V is the potentialof the first terminal of the capacitor 4254, C is the capacitance of thecapacitor 4254, C_(B) is the capacitance of the bit line BL (hereinafteralso referred to as bit line capacitance), and V_(B0) is the potentialof the bit line BL before the charge redistribution. Therefore, it canbe found that assuming that the memory cell 4250 is in either of twostates in which the potentials of the first terminal of the capacitor4254 are V₁ and V₀ (V₁>V₀), the potential of the bit line BL in the caseof holding the potential V₁ (=(C_(B)×V_(B0)+C×V₁)/(C_(B)+C)) is higherthan the potential of the bit line BL in the case of holding thepotential V₀ (=(C_(B)×V_(B0)+C×V₀)/(C_(B)+C)).

Then, by comparing the potential of the bit line BL with a predeterminedpotential, data can be read.

As described above, the semiconductor device illustrated in FIG. 8A canhold charge that is accumulated in the capacitor 4254 for a long timebecause the off-state current of the transistor 4162 is extremely small.In other words, refresh operation becomes unnecessary or the frequencyof the refresh operation can be extremely lowered, which leads to asufficient reduction in power consumption. Moreover, stored data can beheld for a long period even when power is not supplied.

Next, the semiconductor device illustrated in FIG. 8B will be described.

The semiconductor device illustrated in FIG. 8B includes a memory cellarray 4251 (memory cell arrays 4251 a and 4251 b) including a pluralityof memory cells 4250 illustrated in FIG. 8A as memory circuits in theupper portion, and a peripheral circuit 4253 in the lower portion, whichis necessary for operating the memory cell array 4251. Note that theperipheral circuit 4253 is electrically connected to the memory cellarray 4251.

In the structure illustrated in FIG. 8B, the peripheral circuit 4253 canbe provided under the memory cell array 4251. Thus, the size of thesemiconductor device can be decreased.

It is preferable that a semiconductor material of the transistorprovided in the peripheral circuit 4253 be different from that of thetransistor 4162. For example, silicon, germanium, silicon germanium,silicon carbide, gallium arsenide, or the like can be used, and a singlecrystal semiconductor is preferably used. Alternatively, an organicsemiconductor material or the like may be used. A transistor includingsuch a semiconductor material can operate at sufficiently high speed.Thus, the transistor enables a variety of circuits (e.g., a logiccircuit and a driver circuit) which need to operate at high speed to befavorably obtained.

Note that FIG. 8B illustrates, as an example, the semiconductor devicein which the memory cell array 4251 has a stack of the memory cell array4251 a and the memory cell array 4251 b; however, the number of stackedmemory cell arrays is not limited to two. For the memory cell array4251, a stack of three or more memory cell arrays may be used, or onlyone memory cell array may be used.

The transistor 4162 is formed using an oxide semiconductor. Since theoff-state current of the transistor including an oxide semiconductor issmall, stored data can be held for a long time. In other words, thefrequency of refresh operation can be extremely lowered, which leads toa sufficient reduction in power consumption.

A semiconductor device having a novel feature can be obtained by beingprovided with both a peripheral circuit including the transistorincluding a material other than an oxide semiconductor (in other words,a transistor capable of operating at sufficiently high speed) and amemory circuit including the transistor including an oxide semiconductor(in a broader sense, a transistor whose off-state current issufficiently small). In addition, with a structure where the peripheralcircuit and the memory circuit are stacked, the degree of integration ofthe semiconductor device can be increased.

As described above, a miniaturized and highly integrated semiconductordevice having favorable electrical characteristics can be provided.

This embodiment can be implemented in appropriate combination with anyof the structures described in the other embodiments.

Embodiment 4

In this embodiment, a central processing unit (CPU) at least part ofwhich includes any of the transistors disclosed in the above embodimentswill be described.

FIG. 9A is a block diagram illustrating a specific structure of the CPU.The CPU illustrated in FIG. 9A includes an arithmetic logic unit (ALU)1191, an ALU controller 1192, an instruction decoder 1193, an interruptcontroller 1194, a timing controller 1195, a register 1196, a registercontroller 1197, a bus interface (Bus I/F) 1198, a rewritable ROM 1199,and a ROM interface (ROM I/F) 1189 over a substrate 1190. Asemiconductor substrate, an SOI substrate, a glass substrate, or thelike is used as the substrate 1190. The ROM 1199 and the ROM I/F 1189may be provided over a separate chip. Obviously, the CPU illustrated inFIG. 9A is just an example in which the configuration has beensimplified, and an actual CPU may have various configurations dependingon the application.

An instruction that is input to the CPU through the bus interface 1198is input to the instruction decoder 1193 and decoded therein, and then,input to the ALU controller 1192, the interrupt controller 1194, theregister controller 1197, and the timing controller 1195.

The ALU controller 1192, the interrupt controller 1194, the registercontroller 1197, and the timing controller 1195 conduct various controlsin accordance with the decoded instruction. Specifically, the ALUcontroller 1192 generates signals for controlling the operation of theALU 1191. While the CPU is executing a program, the interrupt controller1194 determines an interrupt request from an external input/outputdevice or a peripheral circuit on the basis of its priority or a maskstate, and processes the request. The register controller 1197 generatesan address of the register 1196, and reads/writes data from/to theregister 1196 in accordance with the state of the CPU.

The timing controller 1195 generates signals for controlling operationtimings of the ALU 1191, the ALU controller 1192, the instructiondecoder 1193, the interrupt controller 1194, and the register controller1197. For example, the timing controller 1195 includes an internal clockgenerator for generating an internal clock signal CLK2 based on areference clock signal CLK1, and supplies the internal clock signal CLK2to the above circuits.

In the CPU illustrated in FIG. 9A, a memory cell is provided in theregister 1196. As the memory cell in the register 1196, the memory celldescribed in the above embodiment can be used.

In the CPU illustrated in FIG. 9A, the register controller 1197 selectsoperation of retaining data in the register 1196 in accordance with aninstruction from the ALU 1191. That is, the register controller 1197selects whether data is retained by a logic inversion element or acapacitor in the memory cell included in the register 1196. When dataretention by the logic inversion element is selected, a power supplyvoltage is supplied to the memory cell in the register 1196. When dataretention by the capacitor is selected, the data in the capacitor isrewritten, and supply of the power supply voltage to the memory cell inthe register 1196 can be stopped.

A switching element provided between a memory cell group and a node towhich a power supply potential VDD or a power supply potential VSS issupplied, as illustrated in FIG. 9B or FIG. 9C, allows the power supplyto be stopped. Circuits illustrated in FIGS. 9B and 9C will be describedbelow.

FIGS. 9B and 9C each illustrate an example of a structure of a memorycircuit including any of the transistors described in the aboveembodiments as a switching element for controlling supply of a powersupply potential to a memory cell.

The memory device illustrated in FIG. 9B includes a switching element1141 and a memory cell group 1143 including a plurality of memory cells1142. Specifically, as each of the memory cells 1142, the memory celldescribed in the above embodiment can be used. Each of the memory cells1142 included in the memory cell group 1143 is supplied with thehigh-level power supply potential VDD through the switching element1141. Further, each of the memory cells 1142 included in the memory cellgroup 1143 is supplied with a potential of a signal IN and a potentialof the low-level power supply potential VSS.

In FIG. 9B, any of the transistors disclosed in any of the aboveembodiments is used as the switching element 1141, and the switching ofthe transistor is controlled by a signal SigA supplied to a gateelectrode thereof.

Note that FIG. 9B illustrates the structure in which the switchingelement 1141 includes only one transistor; however, without limitation,the switching element 1141 may include a plurality of transistors. Inthe case where the switching element 1141 includes a plurality oftransistors which serves as switching elements, the plurality oftransistors may be connected to each other in parallel, in series, or incombination of parallel connection and series connection.

Although the switching element 1141 controls the supply of thehigh-level power supply potential VDD to each of the memory cells 1142included in the memory cell group 1143 in FIG. 9B, the switching element1141 may control the supply of the low-level power supply potential VSS.

In FIG. 9C, an example of a memory device in which each of the memorycells 1142 included in the memory cell group 1143 is supplied with thelow-level power supply potential VSS through the switching element 1141is illustrated. The supply of the low-level power supply potential VSSto each of the memory cells 1142 included in the memory cell group 1143can be controlled by the switching element 1141.

In the memory cell described in the above embodiment, data can be heldeven in the case where supply of the power supply voltage is stopped.Therefore, even when the supply of the power supply voltage to theentire CPU including the memory cell group 1143 is stopped asappropriate, the operation speed of the CPU does not become low.Specifically, in the memory cell group 1143, desired data is held evenduring a period in which the supply of the power supply voltage isstopped. Further, at the time of resuming the supply of the power supplyvoltage, the CPU can operate using the held data at once. By stoppingthe supply of the power supply voltage to the CPU in this manner asappropriate, a reduction in power consumption can be achieved.

Although the CPU is given as an example, the transistor can also beapplied to an LSI such as a digital signal processor (DSP), a customLSI, or a field programmable gate array (FPGA).

This embodiment can be implemented in appropriate combination with anyof the structures described in the other embodiments.

Embodiment 5

A semiconductor device disclosed in this specification can be applied toa variety of electronic devices (including an amusement machine).Examples of electronic devices include the following: display devicessuch as televisions and monitors, lighting devices, desktop or laptoppersonal computers, word processors, devices which write data of stillimages or moving images to recording media such as optical discs orreproduce them, audio players, radios, stereos, phones, transceivers,portable wireless devices, cellular phones, game machines, calculators,portable information terminals, electronic notebooks, e-book readers,electronic translators, audio input devices, video cameras, digitalstill cameras, electric shavers, high-frequency heating appliances suchas microwave ovens, electric rice cookers, electric washing machines,electric vacuum cleaners, air-conditioning systems such as airconditioners, dish washing machines, dish drying machines, clothesdryers, futon dryers, electric refrigerators, electric freezers,electric refrigerator-freezers, freezers for preserving DNA, smokedetectors, radiation counters, medical equipment such as dialyzers.Further examples include industrial equipment such as guide lights,traffic lights, belt conveyors, elevators, escalators, industrialrobots, and power storage systems. Note that moving objects driven by aninternal-combustion engine or a motor using electric power are alsoincluded in the category of electronic devices. Examples of the movingobjects include electric vehicles (EV), hybrid electric vehicles (HEV)which include both an internal-combustion engine and a motor, plug-inhybrid electric vehicles (PHEV), tracked vehicles in which caterpillartracks are substituted for wheels of these vehicles, motorized bicyclesincluding motor-assisted bicycles, motorcycles, electric wheelchairs,golf carts, boats or ships, submarines, helicopters, aircrafts, rockets,artificial satellites, space probes, planetary probes, spacecrafts, andthe like. Some specific examples of such electronic devices areillustrated in FIGS. 10A to 10C, FIGS. 11A to 11C, and FIGS. 12A to 12C.

FIG. 10A illustrates a table having a display portion. In a table 9000,a display portion 9003 is incorporated in a housing 9001 and an imagecan be displayed on the display portion 9003. Note that the housing 9001is supported by four leg portions 9002. Further, the housing 9001 isprovided with a power cord 9005 for supplying power.

The transistor described in Embodiment 1 can be used in the displayportion 9003 so that the electronic device can have high reliability.

The display portion 9003 has a touch-input function. When a user touchesdisplayed buttons 9004 which are displayed on the display portion 9003of the table 9000 with his/her finger or the like, the user can carryout operation of the screen and input of information. Further, when thetable is capable of communicating with other home appliances or controlthe home appliances, the table 9000 may function as a control devicewhich controls the home appliances by operation on the screen. Forexample, with the use of a semiconductor device having an image sensingfunction, the display portion 9003 can have a touch-input function.

Further, the screen of the display portion 9003 can be placedperpendicular to a floor with a hinge provided for the housing 9001;thus, the table 9000 can also be used as a television device. When atelevision device having a large screen is set in a small room, an openspace is reduced; however, when a display portion is incorporated in atable, a space in the room can be effectively used.

FIG. 10B illustrates an audio player, which includes, in a main body5021, a display portion 5023, a fixing portion 5022 with which the mainbody is worn on the ear, a speaker, an operation button 5024, anexternal memory slot 5025, and the like. When the transistor describedin Embodiment 1 or the memory described in Embodiment 2 is used in amemory or a CPU incorporated in the main body 5021, power consumption ofthe audio player can be further reduced.

Furthermore, when the audio player illustrated in FIG. 10B has anantenna, a microphone function, or a wireless communication function andis used with a mobile phone, a user can talk on the phone wirelessly ina hands-free way while driving a car or the like.

FIG. 10C illustrates a computer, which includes a main body 9201including a CPU, a housing 9202, a display portion 9203, a keyboard9204, an external connection port 9205, a pointing device 9206, and thelike. The transistor described in Embodiment 1 can be used for thedisplay portion 9203 in the computer. When the CPU described inEmbodiment 4 is used, power consumption of the computer can be reduced.

FIGS. 11A and 11B illustrate a foldable tablet terminal. The tabletterminal is opened in FIG. 11A. The tablet terminal includes a housing9630, a display portion 9631 a, a display portion 9631 b, a display modeswitch 9034, a power switch 9035, a power saver switch 9036, a clasp9033, and an operation switch 9038.

In such a portable device illustrated in FIGS. 11A and 11B, a memory isused for temporarily storing image data or the like. For example, thesemiconductor device described in Embodiment 2 or 3 can be used as amemory. By employing the semiconductor device described in the aboveembodiment for the memory, data can be written and read at high speedand held for a long time, and power consumption can be sufficientlyreduced.

Part of the display portion 9631 a can be a touch panel region 9632 aand data can be input when a displayed operation key 9638 is touched.Although a structure in which a half region in the display portion 9631a has only a display function and the other half region also has a touchpanel function is shown as an example, the display portion 9631 a is notlimited to the structure. The whole region in the display portion 9631 amay have a touch panel function. For example, the display portion 9631 acan display keyboard buttons in the whole region to be a touch panel,and the display portion 9631 b can be used as a display screen.

As in the display portion 9631 a, part of the display portion 9631 b canbe a touch panel region 9632 b. When a keyboard display switching button9639 displayed on the touch panel is touched with a finger, a stylus, orthe like, a keyboard can be displayed on the display portion 9631 b.

Touch input can be performed in the touch panel region 9632 a and thetouch panel region 9632 b at the same time.

The display mode switch 9034 can switch the display between portraitmode, landscape mode, and the like, and between monochrome display andcolor display, for example. The power saver switch 9036 can controldisplay luminance in accordance with the amount of external light in useof the tablet terminal detected by an optical sensor incorporated in thetablet terminal. In addition to the optical sensor, another detectiondevice including a sensor for detecting inclination, such as a gyroscopeor an acceleration sensor, may be incorporated in the tablet terminal.

Note that FIG. 11A shows an example in which the display portion 9631 aand the display portion 9631 b have the same display area; however,without limitation, one of the display portions may be different fromthe other display portion in size and display quality. For example, onedisplay panel may be capable of higher-definition display than the otherdisplay panel.

The tablet terminal is closed in FIG. 11B. The tablet terminal includesthe housing 9630, a solar cell 9633, a charge and discharge controlcircuit 9634, a battery 9635, and a DCDC converter 9636. In FIG. 11B, astructure including the battery 9635 and the DCDC converter 9636 isillustrated as an example of the charge and discharge control circuit9634.

Since the tablet terminal is foldable, the housing 9630 can be closedwhen the tablet terminal is not used. As a result, the display portion9631 a and the display portion 9631 b can be protected; thus, a tabletterminal which has excellent durability and excellent reliability interms of long-term use can be provided.

In addition, the tablet terminal illustrated in FIGS. 11A and 11B canhave a function of displaying a variety of kinds of data (e.g., a stillimage, a moving image, and a text image), a function of displaying acalendar, a date, the time, or the like on the display portion, atouch-input function of operating or editing the data displayed on thedisplay portion by touch input, a function of controlling processing bya variety of kinds of software (programs), and the like.

The solar cell 9633 provided on a surface of the tablet terminal cansupply power to the touch panel, the display portion, a video signalprocessing portion, or the like. Note that the solar cell 9633 can beprovided on one or both surfaces of the housing 9630 and the battery9635 can be charged efficiently. The use of a lithium ion battery as thebattery 9635 is advantageous in downsizing or the like.

The structure and the operation of the charge and discharge controlcircuit 9634 illustrated in FIG. 11B will be described with reference toa block diagram in FIG. 11C. The solar cell 9633, the battery 9635, theDCDC converter 9636, a converter 9637, switches SW1 to SW3, and adisplay portion 9631 are illustrated in FIG. 11C, and the battery 9635,the DCDC converter 9636, the converter 9637, and the switches SW1 to SW3correspond to the charge and discharge control circuit 9634 illustratedin FIG. 11B.

First, an example of the operation in the case where power is generatedby the solar cell 9633 using external light is described. The voltage ofpower generated by the solar cell is stepped up or down by the DCDCconverter 9636 so that the power has a voltage for charging the battery9635. Then, when the power from the solar cell 9633 is used for theoperation of the display portion 9631, the switch SW1 is turned on andthe voltage of the power is stepped up or down by the converter 9637 soas to be a voltage needed for the display portion 9631. In addition,when display on the display portion 9631 is not performed, the switchSW1 is turned off and the switch SW2 is turned on so that the battery9635 may be charged.

Note that the solar cell 9633 is described as an example of a powergeneration means; however, without limitation, the battery 9635 may becharged using another power generation means such as a piezoelectricelement or a thermoelectric conversion element (Peltier element). Forexample, a non-contact electric power transmission module whichtransmits and receives power wirelessly (without contact) to charge thebattery 9635, or a combination of the solar cell 9633 and another meansfor charge may be used.

In a television device 8000 in FIG. 12A, a display portion 8002 isincorporated in a housing 8001. The display portion 8002 displays animage and a speaker portion 8003 can output sound. The transistordescribed in Embodiment 1 can be used for the display portion 8002.

A semiconductor display device such as a liquid crystal display device,a light-emitting device in which a light-emitting element such as anorganic EL element is provided in each pixel, an electrophoresis displaydevice, a digital micromirror device (DMD), a plasma display panel(PDP), or the like can be used in the display portion 8002.

The television device 8000 may be provided with a receiver, a modem, andthe like. With the receiver, the television device 8000 can receivegeneral television broadcasting. Furthermore, when the television device8000 is connected to a communication network by wired or wirelessconnection via the modem, one-way (from a transmitter to a receiver) ortwo-way (between a transmitter and a receiver, between receivers, or thelike) data communication can be performed.

In addition, the television device 8000 may include a CPU for performinginformation communication or a memory. The memory and the CPU describedin any of Embodiments 2 to 5 can be used in the television device 8000.

In FIG. 12A, an air conditioner including an indoor unit 8200 and anoutdoor unit 8204 is an example of an electronic device including theCPU of Embodiment 4. Specifically, the indoor unit 8200 includes ahousing 8201, a ventilation duct 8202, a CPU 8203, and the like. FIG.12A shows the case where the CPU 8203 is provided in the indoor unit8200; the CPU 8203 may be provided in the outdoor unit 8204.Alternatively, the CPU 8203 may be provided in both the indoor unit 8200and the outdoor unit 8204. Since the CPU described in Embodiment 4 isformed using an oxide semiconductor, an air conditioner which hasexcellent heat resistance property and high reliability can be providedwith the use of the CPU.

In FIG. 12A, an electric refrigerator-freezer 8300 is an example of anelectronic device which is provided with the CPU formed using an oxidesemiconductor.

Specifically, the electric refrigerator-freezer 8300 includes a housing8301, a refrigerator door 8302, a freezer door 8303, a CPU 8304, and thelike. The CPU 8304 is provided in the housing 8301 in FIG. 12A. When theCPU described in Embodiment 4 is used as the CPU 8304 of the electricrefrigerator-freezer 8300, power saving can be achieved.

FIG. 12B illustrates an example of an electric vehicle which is anexample of an electronic device. FIG. 12C schematically illustrates theinside of the electric vehicle. An electric vehicle 9700 is equippedwith a secondary battery 9701. The output of power of the secondarybattery 9701 is controlled by a control circuit 9702 and the power issupplied to a driving device 9703. The control circuit 9702 iscontrolled by a processing unit 9704 including a ROM, a RAM, a CPU, orthe like which is not illustrated. When the memory and the CPU describedin any of Embodiments 2 to 5 are used for the electric vehicle 9700,power saving can be achieved.

The driving device 9703 includes a DC motor or an AC motor either aloneor in combination with an internal-combustion engine. The processingunit 9704 outputs a control signal to the control circuit 9702 based oninput data such as data of operation (e.g., acceleration, deceleration,or stop) by a driver or data during driving (e.g., data on an upgrade ora downgrade, or data on a load on a driving wheel) of the electricvehicle 9700. The control circuit 9702 adjusts the electric energysupplied from the secondary battery 9701 in accordance with the controlsignal of the processing unit 9704 to control the output of the drivingdevice 9703. In the case where the AC motor is mounted, although notillustrated, an inverter which converts direct current into alternatecurrent is also incorporated.

Note that any of the electronic devices described above may be directlysupplied with power by a power supply unit such as a solar cell, apiezoelectric element, a thermoelectric conversion element, or anon-contact power transmission module. Alternatively, any of theelectronic devices described above may be supplied with power through apower storage device.

This embodiment can be implemented in appropriate combination with anyof the structures described in the other embodiments.

Reference Example 1

The transistor structure disclosed in this specification is useful for astructure in which a channel formation region is formed in a CAAC-OSfilm. Hereinafter, the feature of an CAAC-OS film that oxygen is easilydiffused in the lateral direction will be described.

Here, as an example of the oxide semiconductor film, ease of excessiveoxygen (oxygen atoms in excess of those in the stoichiometriccomposition) transfer and ease of oxygen vacancy transfer in anIn—Ga—Zn-based oxide (hereinafter, referred to as IGZO) film aredescribed with reference to scientific calculation results.

Note that the calculation was performed in such a manner that models inwhich one excessive oxygen or oxygen vacancy existed in an In—O plane ofIGZO with an atomic ratio of In:Ga:Zn=3:1:2 were made by geometryoptimization (see FIGS. 13A to 13C and FIGS. 15A to 15C), and energy toan intermediate structure along a minimum energy path in each model wascalculated by a nudged elastic band (NEB) method.

The calculation was performed using calculation program software“OpenMX” based on the density functional theory (DFT). Parameters aredescribed below.

As a basis function, a pseudo-atomic localized basis function was used.The basis function is classified as polarization basis sets STO (slatertype orbital).

As a functional,generalized-gradient-approximation/Perdew-Burke-Ernzerhof (GGA/PBE) wasused.

The cut-off energy was 200 Ry.

The sampling point k was 5×5×3.

In the calculation of ease of excessive oxygen transfer, the number ofatoms which existed in the calculation model was set to 85. In thecalculation of ease of oxygen vacancy transfer, the number of atomswhich existed in the calculation model was set to 83.

Ease of excessive oxygen transfer and ease of oxygen vacancy transferare evaluated by calculation of a height of energy barrier Eb which isrequired to go over in moving to respective sites. That is, when theheight of energy barrier Eb which is gone over in moving is high,excessive oxygen or oxygen vacancy hardly moves, and when the height ofthe energy barrier Eb is low, excessive oxygen or oxygen vacancy easilymoves.

First, the movement of excessive oxygen is described. FIGS. 13A to 13Cshow models used in calculation of the movement of excessive oxygen.Note that the longitudinal direction in each of the models correspondsto a c-axis of crystal axes. The calculations of two transfer patternsdescribed below were performed. FIG. 14 shows the calculation results.In FIG. 14, the horizontal axis represents a path length (for themovement of excessive oxygen) and the vertical axis represents energy(which is needed for the movement) with respect to energy in a state ofModel A in FIG. 13A.

Of the two transfer patterns in the case of the movement of theexcessive oxygen, the first transfer is the one from Model A to Model B.The second transfer is the one from Model A to Model C.

In FIGS. 13A to 13C, an oxygen atom denoted by “1” is referred to as afirst oxygen atom of Model A; an oxygen atom denoted by “2” is referredto as a second oxygen atom of Model A; and an oxygen atom denoted by “3”is referred to as a third oxygen atom of Model A.

As seen from FIG. 14, the maximum value (Eb_(max)) of the height Eb ofthe energy barrier in the first transfer is 0.53 eV, and that in thesecond transfer is 2.38 eV. That is, the maximum value (Eb_(max)) of theheight Eb of the energy barrier in the first transfer is lower than thatin the second transfer. Therefore, energy required for the firsttransfer is lower than energy required for the second transfer, and thefirst transfer occurs more easily than the second transfer.

That is, the first oxygen atom of Model A moves in the direction inwhich the second oxygen atom of Model A is pushed more easily than inthe direction in which the third oxygen atom of Model A is pushed.Therefore, this shows that the oxygen atom moves along the layer ofindium atoms more easily than across the layer of indium atoms.

Next, the movement of oxygen vacancy is described. FIGS. 15A to 15C showmodels used in calculation of the movement of oxygen vacancy. Thecalculations of two transfer patterns described below were performed.FIG. 16 shows the calculation results. In FIG. 16, the horizontal axisrepresents a path length (for the movement of oxygen vacancy) and thevertical axis represents energy (which is needed for the movement) withrespect to energy in a state of Model A in FIG. 15A.

Of the two transfer patterns in the case of the movement of the oxygenvacancy, the first transfer is the one from Model A to Model B. Thesecond transfer is the one from Model A to Model C.

Note that dashed circles in FIGS. 15A to 15C represent oxygen vacancy.

As seen from FIG. 16, the maximum value (Eb_(max)) of the height Eb ofthe energy barrier in the first transfer is 1.81 eV, and that in thesecond transfer is 4.10 eV. That is, the maximum value (Eb_(max)) of theheight Eb of the energy barrier in the first transfer is lower than thatin the second transfer. Therefore, energy required for the firsttransfer is lower than energy required for the second transfer, and thefirst transfer occurs more easily than the second transfer.

That is, the oxygen vacancy of Model A moves to the position of oxygenvacancy of Model B more easily than to the position of oxygen vacancy ofModel C. Therefore, this shows that the oxygen vacancy also moves alongthe layer of indium atoms more easily than across the layer of indiumatoms.

Next, in order to compare probabilities of occurrence of theabove-described four transfer patterns from another side, temperaturedependence of each of these transfers is described. The above-describedfour transfer patterns are (1) the first transfer of excessive oxygen,(2) the second transfer of excessive oxygen, (3) the first transfer ofoxygen vacancy, and (4) the second transfer of oxygen vacancy.

Temperature dependences of these transfers are compared with each otherbased on movement frequency per unit time. Here, movement frequency Z(per second) at certain temperature T (K) is represented by Formula (1)when the number of vibrations Zo (per second) of an oxygen atom in thechemically stable position is used.

$\begin{matrix}{Z = {{Zo} \cdot {\exp \left( {- \frac{{Eb}_{\max}}{k\; T}} \right)}}} & (1)\end{matrix}$

Note that Eb_(max) represents the maximum value of the height Eb of theenergy barrier in each transfer and k represents Boltzmann constant inFormula (1). Further, Zo=1.0×10¹³ (per second) is used for thecalculation.

In the case where the excessive oxygen or the oxygen vacancy movesbeyond the maximum value (Eb_(max)) of the height Eb of the energybarrier once per second (in the case of Z=1 (per second)), when Formula(1) is solved for T, the following are obtained.

(1) The first transfer of excessive oxygen: T=206 K (−67° C.) in thecase of Z=1.(2) The second transfer of excessive oxygen: T=923 K (650° C.) in thecase of Z=1.(3) The first transfer of oxygen vacancy: T=701 K (428° C.) in the caseof Z=1.(4) The second transfer of oxygen vacancy: T=1590 K (1317° C.) in thecase of Z=1.

On the other hand, when Formula (1) is solved for Z in the case of T=300K (27° C.), the following are obtained.

(1) The first transfer of excessive oxygen: Z=1.2×10⁴ (per second) inthe case of T=300 K.(2) The second transfer of excessive oxygen: Z=1.0×10⁻²⁷ (per second) inthe case of T=300 K.(3) The first transfer of oxygen vacancy: Z=4.3×10⁻¹⁸ (per second) inthe case of T=300 K.(4) The second transfer of oxygen vacancy: Z=1.4×10⁻⁵⁶ (per second) inthe case of T=300 K.

Further, when Formula (1) is solved for Z in the case of T=723 K (450°C.), the following are obtained.

(1) The first transfer of excessive oxygen: Z=2.0×10⁹ (per second) inthe case of T=723 K.(2) The second transfer of excessive oxygen: Z=2.5×10⁻⁴ (per second) inthe case of T=723 K.(3) The first transfer of oxygen vacancy: Z=2.5 (per second) in the caseof T=723 K.(4) The second transfer of oxygen vacancy: Z=2.5×10⁻¹⁶ (per second) inthe case of T=723 K.

In view of the above-described calculation, excessive oxygen, in thecase of either T=300 K or T=723 K, moves along the layer of indium atomsmore easily than across the layer of indium atoms. Moreover, oxygenvacancy also, in the case of either T=300 K or T=723 K, moves along thelayer of indium atoms more easily than across the layer of indium atoms.

Further, in the case of T=300 K, the movement of excessive oxygen alongthe layer of indium atoms occurs extremely easily; however, the othertransfers do not occur easily. In the case of T=723K, not only themovement of excessive oxygen along the layer of indium atoms but themovement of oxygen vacancy along the layer of indium atoms occurseasily; however, it is difficult for either the excessive oxygen or theoxygen vacancy to move across the layer of indium atoms.

That is, it can be said that in the case where the layer of indium atomsexists in a plane parallel to a surface where a film is formed or asurface of the film (e.g., the case of a CAAC-OS film), excessive oxygenand oxygen vacancy easily move in a parallel direction to the surfacewhere the film is formed or the surface of the film.

As described above, in the CAAC-OS film, excessive oxygen easily movealong a surface where the CAAC-OS film is formed or a surface of theCAAC-OS film. For that reason, in the case where a channel formationregion is formed in a CAAC-OS film in a transistor, diffusion of oxygenfrom the direction horizontal to the channel formation region (from asource region and a drain region) is easier than from the directionperpendicular to the channel formation region (from a base insulatingfilm and a gate insulating film).

Note that the case where the excessive oxygen or the oxygen vacancymoves across the layer of indium atoms is described above; however, thepresent invention is not limited thereto, and the same applies to metalsother than indium which are contained in an oxide semiconductor film.

This application is based on Japanese Patent Application serial no.2012-040837 filed with Japan Patent Office on Feb. 28, 2012, the entirecontents of which are hereby incorporated by reference.

What is claimed is:
 1. A semiconductor device comprising: an oxidesemiconductor layer comprising a source region, a drain region, and achannel formation region over an insulating surface; a gate insulatingfilm over the oxide semiconductor layer; a gate electrode over the gateinsulating film, the gate electrode overlapping with the channelformation region; a source electrode in contact with the source region;and a drain electrode in contact with the drain region, wherein thesource region and the drain region comprise a portion having a higheroxygen concentration than the channel formation region.
 2. Thesemiconductor device according to claim 1, wherein the channel formationregion comprises an oxide semiconductor comprising a c-axis alignedcrystal, and wherein the portion comprises an amorphous oxidesemiconductor.
 3. The semiconductor device according to claim 1, whereinan impurity for improving conductivity of the oxide semiconductor layeris added to the portion.
 4. The semiconductor device according to claim1, wherein at least one of the gate electrode, the source electrode, andthe drain electrode is electrically connected to a semiconductor devicecomprising a semiconductor layer whose band gap is different from a bandgap of the oxide semiconductor layer.
 5. The semiconductor deviceaccording to claim 1, further comprising an insulating film over thegate insulating film and the gate electrode, wherein the insulating filmcomprises aluminum oxide.
 6. The semiconductor device according to claim1, wherein a part of an end portion of the oxide semiconductor layer iscovered with the source electrode and the drain electrode.
 7. Thesemiconductor device according to claim 1, wherein the oxidesemiconductor layer comprises at least one of materials selected fromthe group consisting of indium, zinc, gallium, tin, hafnium, aluminum,and zirconium.
 8. The semiconductor device according to claim 1, whereinthe oxide semiconductor layer comprises a first oxide semiconductorlayer and a second oxide semiconductor layer which are stacked with eachother.
 9. The semiconductor device according to claim 8, wherein thefirst oxide semiconductor layer is located over the second oxidesemiconductor layer, and wherein a concentration of gallium in thesecond oxide semiconductor layer is higher than that in the first oxidesemiconductor layer.
 10. A semiconductor device comprising: a sourceelectrode and a drain electrode over an insulating surface; an oxidesemiconductor layer comprising a source region, a drain region, and achannel formation region over the source electrode, the drain electrode,and the insulating surface; a gate insulating film over the oxidesemiconductor layer; and a gate electrode over the gate insulating film,the gate electrode overlapping with the channel formation region,wherein the source electrode is in contact with the source region andthe channel formation region, wherein the drain electrode is in contactwith the drain region and the channel formation region, and wherein thesource region and the drain region comprise a portion having a higheroxygen concentration than the channel formation region.
 11. Thesemiconductor device according to claim 10, wherein the channelformation region comprises an oxide semiconductor comprising a c-axisaligned crystal, and wherein the portion comprises an amorphous oxidesemiconductor.
 12. The semiconductor device according to claim 10,wherein an impurity for improving conductivity of the oxidesemiconductor layer is added to the portion.
 13. The semiconductordevice according to claim 10, wherein at least one of the gateelectrode, the source electrode, and the drain electrode is electricallyconnected to a semiconductor device comprising a semiconductor layerwhose band gap is different from a band gap of the oxide semiconductorlayer.
 14. The semiconductor device according to claim 10, furthercomprising an insulating film over the gate insulating film and the gateelectrode, wherein the insulating film comprises aluminum oxide.
 15. Thesemiconductor device according to claim 10, wherein the oxidesemiconductor layer comprises at least one of materials selected fromthe group consisting of indium, zinc, gallium, tin, hafnium, aluminum,and zirconium.
 16. The semiconductor device according to claim 10,wherein the oxide semiconductor layer comprises a first oxidesemiconductor layer and a second oxide semiconductor layer which arestacked with each other.
 17. The semiconductor device according to claim16, wherein the first oxide semiconductor layer is located over thesecond oxide semiconductor layer, and wherein a concentration of galliumin the second oxide semiconductor layer is higher than that in the firstoxide semiconductor layer.
 18. The semiconductor device according toclaim 10, wherein the gate electrode overlaps with the source electrodeand the drain electrode.
 19. A method for manufacturing a semiconductordevice, comprising the steps of: forming an oxide semiconductor layerover an insulating surface; forming a gate insulating film over theoxide semiconductor layer; forming a gate electrode over the gateinsulating film so as to overlap with the oxide semiconductor layer; andadding oxygen to a region which is in the oxide semiconductor layer anddoes not overlap with the gate electrode.